Maximum power point tracking of a small scale compressed air energy storage system

This paper is concerned with maximum power tracking of a pneumatically-driven electric generator in a stand-alone small scale compressed air energy storage system (CAES). In this system, an air motor is used to drive a permanent magnet DC generator, whose output is controlled by a buck converter supplying power to a resistive load. The output power of the converter is controlled such that the air motor operates at a speed corresponding to maximum power. The maximum power point tracking (MPPT) controller employs a hybrid perturb and observe method. The rate of change of the converter’s output power with respect to its duty cycle as well as the change of power and duty cycle are used to correct the search direction under transient input power fluctuations. Small speed step changes are used in the vicinity of the maximum power point to improve the accuracy of the search algorithm. However, relatively coarse speed step changes are used when the operating point is far from the MPP to improve the dynamic response of controller and increase its speed of convergence. The analysis and design of the controller is based on a small injected-absorbed current signal-model of the power converter. The controller is implemented experimentally using a real time DSP system. Test results are presented to validate the proposed design and demonstrate its capabilities

Maximum power point tracking of a small-scale compressed air energy storage system

The thesis is concerned with a small-scale compressed air energy storage (SS-CAES) system. Although these systems have relatively low energy density, they offer advantages of low environmental impact and ease of maintenance.The thesis focuses on solving a number of commonly known problems related to the perturb and observe (P&O) maximum power point tracking (MPPT) system for SS-CAES, including confusion under input power fluctuation conditions and operating point dither.A test rig was designed and built to be used for validation of the theoretical work. The rig comprised an air motor driving a permanent magnet DC generator whose power output is controlled by a buck converter. A speed control system was designed and implemented using a dSPACE controller. This enabled fast convergence of MPPT.Four MPPT systems were investigated. In the first system, the air motor characteristics were used to determine the operating speed corresponding to MPP for a given pressure. This was compared to a maximum efficiency point tracking (MEPT) system. Operating at the maximum power point resulted in 1% loss of efficiency compared to operating at the maximum efficiency point. But MPPT does not require an accurate model of the system that is needed for MEPT, which also requires more sensors.The second system that was investigated uses a hybrid MPPT approach that did not require a prior knowledge system model. It used the rate of change of power output with respect to the duty cycle of the buck converter as well as the change in duty cycle to avoid confusion under input power fluctuations. It also used a fine speed step in the vicinity of the MPP and a coarse speed step when the operating point was far from the MPP. Both simulation and experimental results demonstrate the efficiency of this proposed system.The third P&O MPPT system used a fuzzy logic approach which avoided confusion and eliminated operating point dither. This system was also implemented experimentally.A speed control system improved the controllable speed-range by using a buck-boost converter instead. The last MPPT system employed a hybrid P&O and incremental inductance (INC) approach to avoid confusion and eliminate operating point dither. The simulation results validate the design.Although the focus of the work is on SS-CAES, the results are generic in nature and could be applied to MPPT of other systems such as PV and wind turbine

Espaço de viabilidade: uma ferramenta para o desenvolvimento de novas tecnologias de energias renováveis

O objetivo desta Tese é apresentar um espaço de viabilidade como uma meta a ser alçada por pesquisadores e gestores durante o desenvolvimento de uma nova tecnologia de geração ou de armazenamento de energia. Para tanto, utilizou-se o programa HOMER para as simulações, que é um modelo projetado para simular sistemas de energia de pequeno porte, mas que permite, também, a simulação de modelos mais robustos. Primeiramente, é demonstrado um novo estudo de simulação de um sistema híbrido de geração de energia para o litoral Norte do Rio Grande do Sul, com novos custos da energia da rede de distribuição, apresentando o espaço de viabilidade para uma tecnologia genérica de geração de energia de ondas do mar, que ainda se encontra em processo de consolidação. Após, é detalhado o método utilizado para simulação de sistemas híbridos de energia na busca pelo espaço de viabilidade, que compreende um domínio composto por conjuntos de valores de parâmetros chaves, os quais devem ser definidos durante o desenvolvimento de qualquer tecnologia de energias renováveis. Finalmente, é apresentada uma nova aplicação do método proposto, em um novo estudo de sistema híbrido de geração e armazenamento de energia para a barragem de Laranjeiras, localizada em Canela/RS, e com espaço de viabilidade focado para uma tecnologia de armazenamento de energia sob a forma de ar comprimido. Os resultados indicaram que, para as condições consideradas nas simulações, usinas de ondas serão viáveis para a energia da rede variando entre US$0,05/kWh e US$ 0,15/kWh, se puderem ser implementadas com custos entre US$616,01/kW e US$ 2.865,37/kW, respectivamente para eficiências entre 20% e 40%. Esses valores são extremamente baixos, comparáveis aos valores para alternativas já consolidadas, como energia hidrelétrica e eólica. Já para o sistema que inclui a usina de armazenamento com ar comprimido, a solução considerada ótima operou com um custo de energia de US$0,021/kWh e um custo de implementação de US$ 3.023.688,00. Portanto, essa forma de armazenamento permitiu reduzir o custo presente líquido total e o custo da energia gerada pelo sistema.The aim of this thesis is to present a feasibility space as a goal to be achieved by researchers and managers during the development of a new energy generation or storage technology. For that, HOMER program was used for the simulations, which is a model designed to create and simulate small energy systems, but which also allows the simulation of more robust models. First, a new simulation study of a hybrid power generation system for the North coast of Rio Grande do Sul is demonstrated, with new energy costs from the distribution network, presenting the feasibility space for a generic sea wave power generation technology, which is still in the process of consolidation. Afterwards, the method used to simulate hybrid energy systems in the search for the feasibility space is detailed, which comprises a domain composed of sets of key parameter values, which must be defined during the development of any renewable energy technology. Finally, a new application of the proposed method is presented, in a new study of a hybrid energy generation and storage system for the Laranjeiras dam, located in Canela/RS, and with a feasibility space focused on an energy storage technology in the form of compressed air. The results indicated that, for the conditions considered in the simulations, wave plants will be viable for grid energy ranging from US$0.05/kWh to US$ 0.15/kWh, if they can be implemented with costs between US$616.01/kW and US$ 2,865.37/kW, respectively, for efficiencies between 20% and 40%. These values are extremely low, comparable to values for already established alternatives, such as hydroelectric and wind power. For the system that includes the compressed air storage plant, the solution considered optimal operated with an energy cost of US$0.021/kWh and an implementation cost of US$ 3,023,688.00. Therefore, this form of storage made it possible to reduce the total net present cost and the cost of energy generated by the system

A low-cost MPPT multiple-input power converter for home applications in isolated areas

The focus of this research is to design and build a low-cost maximum power point tracking (MPPT) multiple-input-single-output power converter system for low power gridisolated applications. The design of this power converter concentrated on searching for a suitable topology that integrates multiple renewable power sources, each with their own MPPT requirements and with the lowest cost of components. With good power conversion efficiency, the converter provides power to a dc load output and is also able to appropriately charge an energy storage battery. In addition to the main functional blocks, all protections required are equipped with the converter, including under voltage lock-out (UVLO), over voltage protection (OVP), cycle-by-cycle current limit, and battery over charge and over discharge prevention. The development and implementation of the converter was divided by different steps. The initial phase searched for, analyzed, and proposed the most suitable topology in terms of power delivery, cost, and feasibility. The non-isolated full-bridge was chosen for the power conversion topology for each channel with its own analogue controller. An interfacing circuit was designed to work with those full-bridge controllers for integrating MPPT control signals, constant output voltage control signals, and constant charging current control signals, from a microcontroller, a single output voltage feedback loop, and a single output current feedback loop, respectively. After a specification of the design had been selected, the detailed design and calculation of circuits was carried out. Simulations were also conducted to confirm the operation of the converter, including the start-up sequences, output load response, battery charging modes, and the transient between operation modes. The converter was then transferred to PCB design with two versions, a 2-layer and 4-layer board. A comparison was made for choosing the appropriate option among the designs regarding board size, cost, and performance. The final physical converter was formed by soldering components on the manufactured 170mm x 130mm 4-layer PCB, and programming of the microprocessor. The final step was the characterisation of the converter with standard power supplies and renewable energy source emulators, in the laboratory environment. Results show that the converter functions as per the design specifications. With the input source of a solar photovoltaic panel, a micro wind/hydro turbine, or both, the converter can work with solely a dc load or with a dc-bus connected battery (where the converter provides a threestage charging profile). The smooth and fast transiting between operation modes of MPPT, constant output voltage, and constant charging current were recorded without any abnormal behavior. The MPPT functions separately with each input source with high accuracy and fast response to the input conditions. Depending on the state of the output, the converter automatically switches to either constant output voltage or constant charging current mode when the total available input power is higher than the output load demand. For each of these operation modes, the converter also achieves a fast response to the input sources voltage and output load variations. With a designed capability of 2 kW maximum total power conversion, the converter is able to work with a wide range of input voltage (from 16 V to 60 V) for both input sources while its nominal output voltage is set at around 27 V for working with a nominal 24 V lead-acid battery. The peak power conversion efficiency of 95.3 % was recorded at 400 W of total input power when the turbine source voltage was 54 V (dc value after the rectifier) and the solar photovoltaic source voltage was 24 V. The operating temperature of the converter appeared to be higher than expected at some components, with a peak of 68.3 oC recorded on the gate driver chips. However, this issue could be mitigated by adding more heat sink components or modifying the design on a new revision. With under $60 USD of total component cost (slightly above$55 for pre-manufactured components and materials, and about $3 for the PCB), the converter is shown to be a low-cost converter regarding its power capability while supporting multiple input sources which may have a wide range of nominal power outputs. This makes the converter more applicable in isolated areas of developing countries