Abstract
The major challenges in power systems are driven by the energy shortage and environmental concerns, namely facilitating the penetration of renewable energy and improving the efficiency of the renewable sources. Due to the variable nature of renewables, the generated power profile may not be able to match the load requirement. Accordingly, much attention has been focused on the development of energy storage technologies to guarantee renewable power penetration. Recently, advances in the superconducting energy storage system (SMES) have made SMES/battery hybrid energy storage systems (HESS) technically attractive. Compared with other energy storage technologies, the principle advantages of SMES are: the high power density, unlimited cycle-life and high peak current handling capacities. However, SMES has low energy density. The battery is characterised by large energy density, but low power capacity. In renewable energy systems, high-frequency power fluctuations will cause a significant degree of battery power cycling. This, in turn, has been shown to lead to a significant reduction in battery service life. Therefore, the concept of the SMES/battery hybrid energy storage is proposed by combining two kinds of complementary energy storages, achieving an optimised system which has the advantages of both primary energy storage systems (ESS) meanwhile complementing the disadvantages of each ESS.However, the fields of the implementation and optimisation of the proposed hybrid system are relatively new and with challenges remaining in the power management design, the optimal sizing study and the control of the HESS. The main contributions of this project that the thesis introduces are the detailed development and assessment of the SMES/battery HESS with the related power management, system sizing, synergetic control and quantitative evaluation.
To achieve the active combination of two kinds of ESSs, an overall power management integrating the different characteristics of SMES and battery is absolutely essential. Three kinds of power management methods, therefore, are developed to be used in different power applications and analysed in respect of their strengths and weaknesses.
A novel hybrid energy storage sizing method is developed, which is able to return an optimal matching of the SMES and the battery. The different energy and power demands for the battery and the SMES are considered in the sizing method. Moreover, the proposed sizing method is proved to be able to address the SMES oversize problem.
The synergistic control of the battery and the SMES is the key factor to achieve the expected power management in the power systems. A new kind of droop control is, therefore, developed in this study, which effectively co-ordinates the operation of the different storage technologies such that they complement each other. In addition, the droop coefficients for both the SMES and the battery are more carefully determined using the system operating constraints and the energy storage capacities, which makes the control method more efficient.
The proposed SMES and battery hybrid energy storage scheme, together with its control method, is tested by both simulation and experiment. The real-time computing capability of the Real-Time Digital Simulator (RTDS) makes it possible for the HESS system to be interfaced with the outside hardware. To verify the proposed HESS scheme with the new control method experimentally, a hardware-in-the-loop (HIL) test platform is developed coupling with the RTDS. The experimental results show that the presented control method is able to exploit the different characteristics of the ESSs and optimise the power-sharing between the SMES and the battery. Additionally, the battery in the experiment is protected from the continual short-term charge/discharge cycles and abrupt power changes and, as a result, the battery lifetime is improved.
In order to evaluate the improvement of the battery lifespan in the proposed hybrid scheme, a novel battery lifetime model is developed. The new battery lifetime model advances previous ones by taking the battery discharge rate into account, returning a more accurate evaluation. Quantitative analysis of the battery lifetime extension is carried out in this thesis to investigate the long-term performance of the battery in the simulation environment. In the case study, the battery lifetime is extended from 6.2 years in the battery only system to 8.7 years in the SMES/battery HESS, which further highlights the benefits of hybridisation.
| Date of Award | 24 May 2017 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Weijia Yuan (Supervisor) |
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