Investigation and optimisation of hybrid electricity storage systems based on compressed air and supercapacitors

An increasing recourse to renewable energies is one of the key solutions to address the current resource and environmental concerns related to the world energy supply. Because of the distributed and intermittent nature of several of them (Solar, Wind), an efficient and economically viable exploitation of renewable energies relies on the use of energy storage means. Fuel-free compressed air energy storage technologies are highly compatible with renewable energies because of their inherent environmental advantages. However their low energy performances have been the main barrier to their widespread utilization. Pneumatic storage is considered in this thesis with the goal of improving its energetic and power performances so as to make it more efficient and suited for renewable sources support. Storing/generating electrical energy into/from compressed air requires a multiple-step conversion process through an intermediary mechanical energy. Pneumatic-to-mechanical energy conversion is studied first. The suppression of the pressure regulation is proposed to avoid the important energy losses related to this operation. Consequently the volumetric machine must operate at higher and variable pressure. The analysis of the efficiency characteristics of these machines shows the existence of a pressure dependent optimal speed that corresponds to the maximum efficiency. A Maximum Efficiency Point Tracking (MEPT) strategy, based on efficiency-controlled variable speed operation, is proposed for the real time optimization of the conversion efficiency. Experimental results confirm the effectiveness of the proposed strategy both with air machines and oil-hydraulic machines. Oil-hydraulic machines offer higher conversion efficiencies compared to air machines, but require an air-to-oil interface. Two possible ways of realizing such an interface have led to the two hydro-pneumatic storage systems presented. The proposed efficiency-controlled variable speed operation has allowed improving the cycle efficiency of the experimental hydro-pneumatic conversion system by about 4% compared to that of a constant speed operation. In order to provide good power quality and flexibility to these storage systems, a hybrid topology that associates the main, hydro-pneumatic storage subsystem with an auxiliary, supercapacitive storage subsystem is proposed. The power variation is achieved by an intermittent operation of the main storage subsystem and the use of the auxiliary storage subsystem to smooth the resulting power, through the regulation of the common DC bus voltage. The hybrid storage system is thus compatible with a wide range of load and source powers, thanks to the obtained power flexibility. An efficiency analysis shows that the performances of the auxiliary storage greatly affect that of the global storage system. The auxiliary storage should therefore exhibit very high conversion efficiencies so that an acceptable overall efficiency can be expected. A formal method for optimally sizing the supercapacitive auxiliary storage system is proposed, that allows meeting both the voltage and energy requirements while minimizing the cost. A control strategy to optimize the standby efficiency of the interfacing multi-phase DC-DC converter is also proposed, which is based on "power-controlled variation of the number of active phases". Many other application-dependent topologies for the hybrid storage systems are proposed, that help meeting each application's particular requirements while optimizing its performances and cost. A comparative cost evaluation, realized in the context of a stand-alone photovoltaic home application, shows that in addition to its inherent environmental advantages, hydro-pneumatic storage is cost-effective compared to lead acid battery storage

Battery Storage System for Frequency Stabilization of AC Networks with High Penetration of Renewable Power

This paper investigates the uses of battery energy storage system (BESS) in ac networks highly populated with wind power generation. The investigation includes power system load levelling, frequency stabilization, and provision of reactive power support to the wind farm network. The BESS is connected to the wind farm main hub via voltage source converter, while the wind farm is connected to ac network modelled with detailed synchronous generator, including excitation and turbine-governor control. In general the paper attempted to study the role of BESS in modern power system regarding improving system stability. Time-domain simulations conducted in Matlab/Simulink are used to validate the importance of the BESS

Hybrid power management for fuel cell-supercapacitor powered hybrid electric vehicle

Fuel cell (FC) with a combination of supercapacitor (SC) based hybrid electric vehicles have been regarded as a potential solution in the future transportation system. This is due to their zero-emission, enhancement of transient power demand, ability to absorb the energy from the regenerative braking, high efficiency, and long mileage. Nevertheless, the nonlinear output characteristics of the FC system are a feeble point owing to internal constraints such as membrane water content and cell temperature. Hence it is essential to extricate as much power as possible from the stack to avert excessive fuel usage and low system efficiency. Conversely, despite the advantages of the SC as an auxiliary energy storage system, the series connection of SC cells causes a cell imbalance problem due to uneven cell characteristics that occur during the manufacturing process and its ambient conditions. This discrepancy of cell voltages in a supercapacitor module leads to reduce the stack’s efficiency and its lifetime. Furthermore, the above limitations of the power sources and initial state of SC’s charge affect the power management’s distribution of power among the multiple sources. Therefore, the aim of this thesis is to propose a hybrid power management for fuel cell-supercapacitor powered hybrid electric vehicles to solve the three identified problems. Firstly, this thesis focuses on a maximum power point tracking (MPPT) controller with a modified 4-leg interleaved boost converter (M-FLIBC) topology for the FC system. The effectiveness of the proposed IBC with a controller for the FC is compared with the two additional controllers couples with the conventional FLIBC topology. Next, a global modular balancer for voltage balancing of multiple supercapacitor cells is connected in series for an HEV system. The global modular balancing architecture is proposed based on forward conversion, which integrates cell balancing, module balancing, and operating for different frequencies. Thus, greatly reducing the volume and implementation complexity. Finally, the thesis evaluates hybrid power management (HPM) for effective power sources distribution, in order to reduce hydrogen consumption and enhance the vehicle's fuel economy. In this case, an equivalent circuit model of SC is developed for the energy storage system. The combination of an extended Kalman filter (EKF) and traditional coulomb counting (CC) method is used to estimate the SC state of charge in improving the effectiveness of the HPM. To evaluate the fuel economy under realistic driving conditions, the combined environmental protection agency (EPA) test cycles for a city and highway are considered. The outcome of performance comparison of the different controllers based on MPPT technique in terms of voltage, current, power, settling time, and efficiency of the FC indicates that the radial basis function network (RBFN) based MPPT controller with the M-FLIBC outperforms the PID and Fuzzy based controllers. With respect to controlling of SC in HEV environment, the proposed topology of SC presents effective voltage balancing with a lower component count, able to operate at different frequencies, i.e., 10 to 70 kHz, as well opens to unlimited stackable modular numbers of SC cells for the HEV performance analysis. Ultimately, with all the proposed control topologies and combined EKF-CC based power management for the FC-SC in Series HEV, the vehicle's fuel economy is increased to 93.38 km/kg as compared to traditional CC based power management of 86.53 km/kg, besides it improves the vehicle’s acceleration within 0-100 km/h in 9.0 seconds respectively. Finally, the research shows that the hybrid power management of FC and SC powered HEV leads to improved performance of the vehicle in terms of the key measures. Suggestions for future research are also highlighted

PSO-based PID controller design for an energy conversion system using compressed air

U ovom se radu predlaže optimalni kontrolni algoritam za rješavanje problema niske performanse zbog nelinearnih značajki pneumatskog motora u sustavima za pretvorbu energije pomoću stlačenog zraka. Učinkovitost predloženog algoritma se ispituje na sustavu za pretvorbu energije koji uključuje kompresor, proporcionalni ventil, pneumatski motor (PM), generator istosmjerne struje s trajnim magnetom (PMDC) i kontrolnu karticu. Kontrolna funkcija sustava provodi se pogonjenjem proporcionalnog ventila s kontrolnim signalima što se postiže ovisno o greški napona na izlazu PMDC generatora. U toj konstrukciji, optimalni proporcionalni-integralni-derivativni (PID) regulator izravno podešava vlastite parametre pojačanja algoritmom optimizacije roja čestica - particle swarm optimization (PSO) u skladu s radnim uvjetima primijenjenog sustava. U svrhu promatranja učinaka PID-regulatora zasnovanog na PSO na rad sustava, sustav za pretvorbu energije se kontrolira PID regulatorom diskretnog vremena. Eksperimentalni rezultati pokazuju da PID regulator zasnovan na PSO osigurava robustniju regulaciju rada nego PID regulator diskretnog vremena kod različitih radnih uvjeta.In this study, an optimal control algorithm is proposed to overcome low performance problems arising from the non-linear characteristics of pneumatic motor in compressed air-based energy conversion systems. The effectiveness of the proposed algorithm is tested on an energy conversion system which includes a compressor, a proportional valve, a pneumatic motor (PM), a permanent magnet direct current (PMDC) generator and a control card. The control function of the system is carried out by driving the proportional valve with the control signals which is obtained depending on the PMDC generator output voltage error. In this structure, an optimal proportional-integral-derivative (PID) controller which tunes on-line its own gain parameters by particle swarm optimization (PSO) algorithm according to the operating conditions of the system used. In order to observe the effects of PSO-based PID controller on the system performance, the energy conversion system is also controlled by a discrete time PID controller. The experimental results show that PSO-based PID controller provides more robust control performance than discrete time PID controller under various operating conditions

Power Management Strategy of a Fuel Cell Hybrid Electric Vehicle with Integrated Ultra-Capacitor with Driving Pattern Recognition

abstract: The greenhouse gases in the atmosphere have reached a highest level due to high number of vehicles. A Fuel Cell Hybrid Electric Vehicle (FCHEV) has zero greenhouse gas emissions compared to conventional ICE vehicles or Hybrid Electric Vehicles and hence is a better alternative. All Electric Vehicle (AEVs) have longer charging time which is unfavorable. A fully charged battery gives less range compared to a FCHEV with a full hydrogen tank. So FCHEV has an advantage of a quick fuel up and more mileage than AEVs. A Proton Electron Membrane Fuel Cell (PEMFC) is the commonly used kind of fuel cell vehicles but it possesses slow current dynamics and hence not suitable to be the sole power source in a vehicle. Therefore, improving the transient power capabilities of fuel cell to satisfy the road load demand is critical. This research studies integration of Ultra-Capacitor (UC) to FCHEV. The objective is to analyze the effect of integrating UCs on the transient response of FCHEV powertrain. UCs has higher power density which can overcome slow dynamics of fuel cell. A power management strategy utilizing peak power shaving strategy is implemented. The goal is to decrease power load on batteries and operate fuel cell stack in it’s most efficient region. Complete model to simulate the physical behavior of UC-Integrated FCHEV (UC-FCHEV) is developed using Matlab/SIMULINK. The fuel cell polarization curve is utilized to devise operating points of the fuel cell to maintain its operation at most efficient region. Results show reduction of hydrogen consumption in aggressive US06 drive cycle from 0.29 kg per drive cycle to 0.12 kg. The maximum charge/discharge battery current was reduced from 286 amperes to 110 amperes in US06 drive cycle. Results for the FUDS drive cycle show a reduction in fuel consumption from 0.18 kg to 0.05 kg in one drive cycle. This reduction in current increases the life of the battery since its protected from overcurrent. The SOC profile of the battery also shows that the battery is not discharged to its minimum threshold which increasing the health of the battery based on number of charge/discharge cycles.Dissertation/ThesisMasters Thesis Mechanical Engineering 201

Hybridizing Lead-Acid Batteries with Supercapacitors: A Methodology

Hybridizing a lead–acid battery energy storage system (ESS) with supercapacitors is a promising solution to cope with the increased battery degradation in standalone microgrids that suffer from irregular electricity profiles. There are many studies in the literature on such hybrid energy storage systems (HESS), usually examining the various hybridization aspects separately. This paper provides a holistic look at the design of an HESS. A new control scheme is proposed that applies power filtering to smooth out the battery profile, while strictly adhering to the supercapacitors’ voltage limits. A new lead–acid battery model is introduced, which accounts for the combined effects of a microcycle’s depth of discharge (DoD) and battery temperature, usually considered separately in the literature. Furthermore, a sensitivity analysis on the thermal parameters and an economic analysis were performed using a 90-day electricity profile from an actual DC microgrid in India to infer the hybridization benefit. The results show that the hybridization is beneficial mainly at poor thermal conditions and highlight the need for a battery degradation model that considers both the DoD effect with microcycle resolution and temperate impact to accurately assess the gain from such a hybridization