ENERGY STORAGE MICHAEL LIPPERT SAFT Microgrids make a powerful case for Li-ion energy storage Michael Lippert of SAFT outlines the growing trend for microgrids to incorporate lithium-ion (Li-ion) energy storage systems. Communities and industrial facilities in remote off-grid locations or where the local grid is unreliable have historically relied on diesel generators for a secure source of electricity. However, they are now turning to more sustainable renewable energy resources, and especially solar photovoltaic (PV) installations. Hence the growing trend for microgrids that combine a hybrid of diesel generation and PV plant with an energy storage system (ESS). This approach offers considerable savings in terms of the costs of fuel purchase, transport and handling as well as maintenance, since the diesel gensets are required to operate for fewer hours. There are also significant environmental benefits from both reduced greenhouse gas emissions and quieter operation. The cost of solar and wind per kilowatthour is significantly lower than diesel generation. However, the inherent intermittency and steep ramp-rate of renewables means that diesel gensets remain essential for grid stability. With standard power electronics, PV can deliver up to 20 to 30 percent of the power output of the diesel genset during daytime hours. By adding dedicated software the penetration of PV can rise by 50 to 60 percent. The introduction of energy storage can make it possible to increase penetration and harvest all of the PV output, with realistic fuel savings in the range of 50 to 75 percent. For megawatt scale energy storage, Liion battery systems have emerged as the technology of choice due to their high energy density that packs significant storage capacity within a compact footprint. Integrated containerized systems can be connected in parallel to deliver multiple megawatt-hour storage capacity. The three drivers for energy storage From a technical and economic point of view, energy storage has three drivers. Firtst–increasing PV utilisation reduces the diesel generator running hours. Without energy storage, excess PV would be lost (or curtailed) but the ESS can store it for later use. In addition, the ESS smoothes the power output of the PV panels, so that the diesel generator needs to start and stop less often to compensate for the variation in PV. Second–the diesel generator runs at its point of maximum efficiency, rather than ramping up and down to meet changing demand. This reduces operational and maintenance costs as well as idle running to provide spinning reserves. Third – the diesel generator is only required to operate when needed to support loads or to charge the battery. How does energy storage work within a microgrid? Consider an industrial microgrid with a 12 MW load. Typically, this would be supported by six diesel generators, each rated at 2 MW. SMA has modelled the impact of adding 60 to 150 percent of PV production (between 7.5 and 18 MWp) together with energy storage to fulfil the roles of power smoothing and time shifting. For power smoothing a relatively small ESS is required to deliver power for 20 minutes to compensate for changing weather conditions. This delivers savings by avoiding the need to ramp the diesel generators up and down. However, they are still required for spinning reserves and battery recharging at times of low PV production is low. Without an ESS, incremental savings can be achieved by incorporating additional PV. But the ESS helps to realise significantly more savings. In fact, for power smoothing a medium sized battery is a better option for optimizing PV penetration than adding extra panels. A battery sized for about 40 percent of the system power (4.6 MW) and 20 minute discharge is sufficient to enable a 50 percent decrease in fuel consumption. This could save up to an estimated four million litres of diesel per year in combination with PV panels sized between 7.5 and 11.5 MWp. Any further increase in battery power would only deliver marginal additional fuel savings. For time shifting, a larger battery would supply two hours of energy storage so that PV energy generated during the peak hours of daylight can be used during the hours of peak demand in the morning and evening. The larger battery enables greater levels of PV to be integrated. At up to 18 MWp of additional PV, this represents up to 54 energética INTERNATIONAL · Nº 158 · JUN/JUL16
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