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systems. System operators measure the balance by monitoring system frequency. In Europe, the system target is a grid-level frequency of around 50 Hertz (Hz); in North America it is 60 Hz. Failure to operate the system at its required frequency can disrupt the operation of equipment, disconnect power plants to prevent damage and lead to large-scale blackouts. Increasing amounts of fluctuating renewables thus place a greater emphasis on grid flexibility, which ensures electricity supply reliability. Fossil fuel power plants and interconnectors provide most of the necessary flexibility at the moment to maintain system frequency. The use of more renewable energy therefore draws greater attention to any available alternative. A range of facilities play an important role in providing electricity system flexibility. These include dispatchable plants, grid transmission lines to connect supply and demand (interconnection), energy storage, and demand side measures such as distributed generation. These sources of flexibility must be evaluated for individual systems given the diverging characteristics of various parts of the world The technical term for describing the ability of an electricity system to resist changes in frequency is inertia. It is determined by the characteristics of the generators and loads in a system. Broadly, this is understood through the degree of spinning masses and motors synchronised to system frequency. Low inertia can be expected in a small system such as an island with limited interconnection and few power plants. On the other hand, interconnected grids with ample generation assets, such as the German network, have high inertia. Systems with high inertia recover more quickly from initial frequency changes stemming from unexpected supply and/or demand deviations. In large interconnected systems the first instance of response to frequency changes occurs automatically and immediately, and is known as governor control. Synchronous generators (power plants whose rotational speed is synchronised to grid frequency, i.e. 60 Hz) generally have some capacity set aside to respond to sudden changes in system frequency. The collective action of synchronous generation governor control has the ability to oppose frequency changes automatically, and is available to the system within seconds. Wind turbines and solar PV are not synchronous generators, but can mimic synchronous generation with power electronics. This issue takes on greater importance at high levels of variable renewable energy penetration. Although renewables can provide the same functions as synchronous generators, there are no examples yet of large interconnected systems that are balanced through renewable power. Implementation planning and physically interconnecting generation to demand centres helps reduce the necessity for additional flexibility. For instance, renewable generation geographically dispersed and interconnected across a larger area allows less variability in supply. This is also true of wind and solar generation. Wind and solar are also able to incorporate power electronics and storage to provide automatic adjustments. These mimic traditional power plants, according to the U.S National Renewable Energy Laboratory. Storage may be essential to reliably integrate power generated from renewable energy in systems that have weak interconnection. Dispatchable plants, which can be called upon to increase or decrease electricity production, have traditionally been driven by fossil fuels. However, battery storage may mitigate frequency deviations at the grid level. It can also make variable renewables more dispatchable by storing excess electricity production on site. Energy storage consists of a suite of technologies at various stages of development. The most mature energy storage technology is pumped hydropower, generally utilized for longer periods of charge and discharge (multiple hours). Pumped hydropower represents the vast majority (99%) of storage in use. It is economically and technically proven throughout the world. By contrast, battery storage is a new market development. Examples of other emerging storage technologies are adiabatic compressed air energy storage, flywheels, power to gas and super capacitors. Electricity can also be stored in thermal form using boilers, heat pumps, ice or chilled water, for instance. Thermal storage can be integrated with combined heat and power production and utilised to maximise wind resource penetration. Thermal energy storage options are often cheaper than other forms of storage, though it is more difficult to reverse heat storage back into electricity. Typically, electric energy converted to a thermal medium is used at another time as thermal energy, either for space heating, cooling or in industrial processes. The types of batteries discussed in this report are secondary (rechargeable) batteries, unlike the non-rechargeable batteries used in some consumer applications. These batteries store energy chemically. They are low temperature (lithium-ion, lead-acid, nickelcadmium), high temperature (sodium nickel chloride, sodium-sulphur) or redox flow (vanadium, zinc bromine). Component materials are sourced from various locations around the world, and their availability or scarcity has an impact on the cost and sustainability of the battery – see box 4. Battery storage is one option that can mitigate both the short (defined here as seconds) and long-term (defined here as several hours) fluctuation of renewable energy. It does this through several different applications and locations in the electricity system, including battery storage in distribution networks or households. Batteries are generally not suited to medium and longer-term or seasonal storage lasting several months. Battery storage in the power sector needs to overcome many barriers before it can be integrated as a mainstream option. One barrier is the lack of monetary Variability in electricity supply must be accounted for to maximise renewable energy penetration into the electricity system and ensures a match between electricity supply and demand at all times. Modularity is another characteristic of some renewable energy types, especially PV and wind RENEWABLE ENERGY 50 energetica INDIA · MAY | JUN16


energetica-india-57_asiapowerweek
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