Investigation and optimisation of hybrid electricity storage systems based on compressed air and supercapacitors (2006)

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.