Projects:2019s1-180 Nanogrid Development for Households Applications

Members & Supervisors: Supervisor - Nesimi Ertugrul Wassim Saad Dehong Wang Shuting Dai

Abstract:

Background:

What is a grid?

An electrical grid (macrogrid) is electrical power system network which consists of a generating plant, transmission lines, the substation, transformers, the distribution lines, and the person who consumes the electricity. Typically, electrical grids have depended on a central generation plant which usually far away from the main consumption areas, being towns and cities. The electrical grid can be broken down into 3 subsections. It consists of: Generation, Transmission & Distribution, and Consumption. The generation component of the electrical grid can be classified as either centralized generation, or decentralized generation. Centralized generation consists of a large electrical production plant, which distributes electricity to the consumers via the grid. Centralized generation may include coal, nuclear, natural gas, hydropower, wind farms, and large solar arrays. Decentralized generation is generated power that occurs close to consumption, such as rooftop solar. Once the electricity has been produced in the generation stage, it must pass through the transmission & distribution component in order to reach the consumers. This component consists of transformers, substations, and transmission lines which transmit the electricity over long distances. After the transmission & distribution stage, the electricity reaches the consumption stage, where the electricity is consumed by the consumers at their demand. The goal of an electrical grid is to safely and stably distribute electrical power, based on consumer’s demand, at prescribed levels of security, whilst ensuring system frequency and voltage levels remain relatively constant. Managing an electrical grid consists of several challenges, which arise from managing the supply of electricity production to match consumer demand, as well as ensuring that the voltage and frequency remain constant at prescribed levels across the entire grid. In recent years, maintaining the constant frequency and voltage levels has become increasingly challenging due to the rapid integration of renewable energy sources. Before the introduction of renewable energy to the electrical grids, grids relied on a non-renewable power source to produce the electricity. Maintaining constant frequency and voltage is simplified with these non-renewable sources due to them being synchronous generators - which converts mechanical power from a prime mover into an AC electrical power at a particular voltage and frequency. The constant speed of the generator - called the synchronous speed - results in the constant voltage and frequency output, and hence allows for energy management to be accurately managed, allowing for grid reliability and stability. On the other hand, renewable energy sources particularly wind and solar energy, can have fluctuating output. Wind turbine power output may fluctuate due to wind speed rarely being constant, and with the tendency to completely drop off at any given moment. The following graph demonstrates the typical output of a wind turbine over a one week period:

The fluctuating wind speed leads to a variety of rotational speeds of the wind turbine, which impacts its output, as different speed correspond to different frequency voltage levels. The output of a solar photovoltaic cell may also fluctuate due to clouds impacting solar intensity, which also leads to a unpredictable output. This can be shown with the following curve of the output of a PV cell where clouds were present:

As shown by the above two curves, the output of both wind and solar is very unpredictable. Around the world, the integration of renewable energy into electrical grids - through either centralized systems (wind/solar farms) or distributed generation (rooftop solar) - has led to significant difficulties with managing the power system due to these exact fluctuations in output, leading to unstable frequency, hence a degree of unreliability. This case of management difficulty is relevant to South Australia, due to the SA government making a long term commitment of maximising renewable energy output, with the eventual goal of achieving 100% renewable. This led to South Australia closing down the Northern Power Station - SA’s largest coal power plant - making the main source of electricity production renewable energy. This led to significantly high electricity prices, and reliability issues. These management difficulties have lead to the potential need to research different grid options so that renewable energy can become a reliable and dependable source for the future. This is where smaller scale grids, such as microgrids and nanogrids, become significant. A microgrid is defined as a small network of electricity users with a local source of supply that is usually attached to a centralized national grid, but is able to function independently. A microgrid is essentially the same as the macrogrid, but at a communal level. The same definition of this applies to nanogrids, except that nanogrids typically applies for a single building, whether it be a household or office building. In order to completely integrate renewable energy to become a dependable source, the issue of reliability and costs must be solved. Currently, there are a few different methods that grids largely dependent on renewable sources use. One method is the use of large scale battery storage. Battery storage was introduced to South Australia’s grid, when Tesla-Neoen constructed a 100 MW lithium-ion grid support battery. The aim of implementing the large scale batteries was to improve the stability of the SA grid, which experienced a significant amount of instability prior to the battery’s construction. The goal of the battery’s construction was to identify if the grid could become fully dependent on renewable energy with the inclusion of energy storage, as a precursor for the rest of the world. Since its implementation, the battery has demonstrated success in stabilising grid frequency and managing the RoCoF. The similarities this has to the proposed nanogrid system is the dependence the grid has on renewable energy sources, as well as integrating it with battery storage. The key difference to the proposed nanogrid system is the use of battery storage to maintain stability of the grid frequency. The proposed nanogrid project aims to use the battery storage as a backup energy source, rather than frequency management.

Introduction: