Kinetic Energy Storage Systems for Islanded Grids
The cost of electricity in rural Alaska is four to ten times higher than the national average, with diesel fuel as the primary fuel source for most isolated microgrids. However, with advancements in renewable energy technology, an increasing number of rural communities are supplementing diesel generators with renewable energy resources. Where implemented well, use of renewable energy leads to reduced diesel consumption as well as reduced CO2 emissions.
In isolated grids, diesel generators have two functions: to provide electric energy and to help with grid stabilization. Diesel generators are kept online for spinning reserve so that added capacity is always available to buffer sudden reductions of renewable energy production or increases in demand. However, providing spinning reserve capacity with diesel generators necessitates operation at reduced load factors, which results in reduced efficiency of the diesel plant. This diminishes the cost effectiveness of the renewable energy systems and limits the maximum penetration of renewable energy on the grid to levels where the diesel generators can economically and efficiently balance changing supply with demand.
An alternative way to stabilize these grids and allow for a higher share of renewable energy is through the use of an electric energy storage system (EESS). Its basic function is to store energy when there is a surplus available and to release energy when there is a deficit. In islanded grids, EESS can replace online generators in providing spinning reserve for just long enough to bring diesel generators back online. High power density and a practically unlimited life cycle make kinetic energy storage systems (KESS) well suited for this application. KESS would allow the grid to meet the same demand with a lower capacity of diesel generators, leading to an ultimate reduction in diesel fuel consumption.
This project used the specification, design and assessment methodology developed at the Institute for Mechatronic Systems at the Technical University Darmstadt, Germany to size a KESS for the islanded grid of Nome. In order to optimize grid stability and to reduce actual energy costs, the operational strategy of the EESS had to be carefully analyzed to meet the characteristics of the specific islanded grid.
Based on a data-driven time-series energy balance model developed by the Alaska Center for Energy and Power, technology neutral requirements were derived in the “specification” step. In the “design” phase, a KESS was designed to meet these requirements. An operational strategy was determined, and in the “assessment” step, the KESS was simulated and analyzed to ensure its functionality and economic feasibility.
An outer-rotor flywheel developed at Technical University Darmstadt was modeled for optimal power and capacity to the requirements of Nome utilities. The resulting conceptual storage system consists of seven KESS, which provide 959 kW of total power with a combined capacity of 58 kWh.
The simulations showed that the KESS is an efficient way to provide spinning reserve capacity in high wind periods. The storage system reduced the diesel consumption during those periods by 740 to 950 gallons per week. In lower wind periods, the diesel savings were 317 to 580 gallons per week. The implemented operational strategy of the storage system does not influence the scheduling of the diesel generators in very low-wind periods. Consequently no reduction of the diesel consumption was observed during these periods; in a few cases, the energy demand of the storage system also led to a slight increase in diesel consumption. Load smoothing was implemented into the operational strategy of the storage system as a secondary function, relieving the diesel generators from high-dynamic load changes.
Left Photo: Street in City of Nome. Courtesy of ACEP.
Middle Photo: Hatch Flywheel in the ETF at ACEP. Courtesy of ACEP.
Right Photo: Nome Windmill Installation. Courtesy of ACEP.