Inage, S. 2009. Prospects for Large-Scale Energy Storage in Decarbonized Power Grid. International Energy Agency.
This paper limits itself to the issue of frequency stability in systems with increasing shares of variable renewable generation assets (wind power in Western Europe (WEU) goes from 9.8% now to 25.4% in 2050, etc see page 23)
Electric frequency is controlled within a small deviation: for example, in Japan the standard is 0.2-0.3 Hz; in the U.S. it is 0.018-0.0228 Hz; and in the European UCTE it is 0.04-0.06Hz. As renewables increase, the potential for fatal frequency changes grows, since such generators rarely have frequency control systems and can produce large variations in output as weather conditions change.
The need to ensure supply that matches demand under all circumstances poses particular challenges for variable renewable power options such as wind and solar generation, whose supply heavily depends on season, time and weather conditions. Short-term variations are quite random and difficult to forecast.
Existing regional grids with high shares of variable renewable do not always provide a relevant reference for a future power system with high share s of renewables. The reason is that such grids do not operate as islands; rather, they are well connected to other grids that stabilize their operation.
This is the case for Denmark and Northern Germany. In 2001, the demand and supply of wind power corresponded fairly closely. When excess power was available, it could be exported through interconnections with Norway, Sweden and Germany. Conversely, power could be imported in periods of shortfall. Therefore, in Denmark, no counter measure would be needed to mitigate short-term and long-term variations, despite an anticipated greater share of wind power. Interconnectors provide a key short-and medium-term option to deal with the variability of renewable power generation, but will not be sufficient to deal with large grids on a continental scale with high renewables penetration.
This paper looks at what’s needed if wind power and solar power provides 12% and 11% of global electricity generation by 2050.
Variable output renewable technologies such as wind and solar are not dispatchable.
The variability characteristics of solar, wind and impoundment hydro power vary substantially from season to season, day to day, time to time. Wind turbines may be shut off during storm conditions that could last for hours. Wind speeds may fall to zero or very low levels over large areas for days. Solar power is not generated at night, and insolation levels may be significantly reduced in winter, especially at higher latitudes. Solar power may also fluctuate depending on cloud levels and the moisture content of the air. Finally, hydro power may be absent in dry years, depending on the water inflow (glaciers or rainfall). These different variability characteristic require different types of response strategies.
With large shares of these technologies, steps would need to be taken to ensure the continued reliable supply of electricity. While related issues include voltage and frequency variations, this report focuses on frequency stability. Constant balance of demand and supply is essential to achieve this, and, in the majority of today’s power systems, mid load technologies such as coal and gas and in some cases hydro, play the chief role in this regard.
The main focus of this paper is to investigate the storage growth and total global storage capacity needed between 2010 and 2050, to assist in the balancing of power systems with large shares of variable renewables.
Variable renewable energies are associated with weather-related power output variations, which consist of short term variations on a scale of seconds to several minutes, superimposed on long term variation on the scale of several hours. Frequency change depends on the short-term variation, therefore this report focuses on short–term variations.
Although the output of individual wind or solar plants can vary considerably, wide geographical dispersal of wind power and PV plants reduces the net variation of many plants as seen by the system as a whole. The net output variation of renewables is an important parameter in this analysis. To date, the impact of this smoothing effect varies from region to region. If the outputs of individual wind and PV plants are uncorrelated, the extent of variation decreases with the inverse square root of the overall number of plants. On the other hand, over relatively small areas with large numbers of wind and PV plants, plants may show strong correlation with each other. In such situations a significant net variation will remain.
The extent to which a power system can accommodate variations in supply is governed to a large extent by its flexibility–a measure of how fast and how much the system can quickly increase or decrease supply or demand, to maintain balance at all times. A range of measures exist to increase the flexibility of power systems, and thus the extent to which they can accommodate variable renewables. This paper looks at one of these measures–storage.
Another option is to interconnect among adjacent power systems. For instance, in Western Europe (WEU), interconnected power grid and electricity trading play an important role.
Flexible power plants such as gas and hydro can act as reserves to provide for deficits in wind power generation across the interconnected area, while at the same time the geographic smoothing effect is increased because the total area is larger. At present, in Denmark, where the average share of wind power is approximately 20%, effective balancing of supply and demand is facilitated through electricity trade with other Scandinavian countries.
However, taking for example a cluster of interconnected systems lying under a single weather system, all with a high share of variable renewables, trade of electricity may not be relied upon for fast access to additional electricity during low wind / solar periods, nor to dispose of surpluses, because deficits and surpluses among all such systems will coincide to a large extent. Moreover, reduced flexible power plant capacity over the entire region in 2050, due to partial displacement by renewables and nuclear, may lead to a lack of flexible reserves. To provide for such cases, internal solutions need to be in place. Balance will not be maintained by interconnectors alone, and system designers and operators should look at additional measures such as energy storage.
Simulations of wind power variation levels between 5% and 30% yield estimates of energy storage capacity in the WEU ranging from 0 GW to 90 GW in 2050. The balance between the demand and the supply was calculated for every 0.1 hr (6 minutes). To estimate energy storage worldwide, net variations were assumed as 15% and 30%. Simulations undertaken suggest that a worldwide energy storage capacity ranging from 189 GW to 305 GW would be required.
As mentioned above, as each storage system has different specifications, the optimal arrangement of these systems depends on circumstances in individual countries. In Annex 1, the current technical potential of NaS cells, pumped hydro, redox flow cells, Compress ed Air Energy Storage (CAES), electric double-layer capacitors, Li-ion batteries, Superconducting Magnetic Energy Storage (SMES) and flywheel systems is reviewed. Reducing costs of such storage technologies may be a key to expanding the use of energy storage technologies to keep pace with the growth of variable renewables.
Grid Operation and Load Curves
Load duration curves can be split into base and peak loads. Base loads are generated by plants whose output is difficult to change; they therefore operate most of the time at full capacity. Base loads are generally served by either high-efficiency fossil-fired or nuclear reactor power plants with low production cost. Peak loads are usually served by natural gas combined-cycle plants, gas turbine generation, or hydropower plants that can change their output in a short time, although with high production cost.
An interesting case of a power system with a high proportion of wind power is found in Spain and Portugal on the Iberian Peninsula. In 2008, there was a day when the share of wind power in the total power supply reached 23% in Spain. This high proportion created power quality problems that have since been resolved through better interconnect ion s within Spain. In addition, Spain has significant pumped hydropower capacity that can mitigate power supply variation s during the operation.
It is preferable that wind power generation resources be distributed to maximize the smoothing effect, which is the key to reducing net variation of the wind power supply. Since the necessarily capacities of energy storage depend on the net variation of wind power, measuring methods and analytical systems should be established by individual countries or groups of countries. Through an accumulation of these efforts , the necessary countermeasures should be determined