Introduction
Renewable resources such as wind, solar, and hydro are abundant at some times and in some places, but they are not always available when and where they are needed. Other energy sources are required to allow these renewable resources to be used in utility electrical grids, in the form of storage or dispatchable generation technology that can be used when required. Managing the intermittent nature of renewables is generally not a problem so long as the fraction of this power is less than 10% of the total energy in the system.
The use of energy storage is considered by many as an essential component of future utility delivery infrastructure throughout the United States as more intermittent renewable energy is added to the system. Energy storage can be used in a wide range of applications to improve availability and reliability of delivered power, to support variable distributed generation (including renewables), to stabilize transmission and distribution lines, or to time shift consumption through bulk storage to achieve the most efficient use of baseload generation. Many of these applications require short bursts of power to balance and control energy, with discharge times ranging from milliseconds to a few minutes.
Energy Storage Options
System configuration and design philosophy will largely determine the incorporation of storage. Storage time can range anywhere from a long-term seasonal or annual basis, down to an hourly or even shorter basis.
It should also be noted that in grid connected applications storage is not an end in itself, but a bridge used as required to insure power quality or to simplify control. Storage is used to bridge a low-cost energy option like hydro or wind, to a higher-cost energy option like natural gas or diesel, allowing a seamless switch between the two. Long-term storage is typically not economical. When compared, the capital and maintenance costs of storage technology are well above the cost of providing power with conventional dispatchable generation.
However, rural Alaskan communities are not connected to larger power grids, and recent increases in diesel electric power generation have led to increased interest in wind power. Current commercial wind turbines are often larger than the load in these small communities, but the variability of the wind make it very difficult to balance these systems with diesel engines. Current systems use electrical dump loads to stabilize the system, but electrical storage would be preferred. New battery technologies are being developed that, if proven, might provide reliable power from wind at costs in range with current diesel generation costs.
Many strategies can be used for energy storage, including chemical storage (hydrogen, electrochemical (batteries), electrical (capacitors), mechanical (flywheels or pumped hydro), or thermal storage. In grid connected situations, only pumped-hydro is economic, but other storage is used for increased reliability for some applications, such as batteries for UPS systems for computers. All electrical storage systems add flexibility to the electric grid, increasing the options available for grid optimization and management. Which storage technology would perform best depends on local economics, the proposed application, and details of the site.
Storage can be classified by several key parameters:
- Rated Power (kW) The rated power output available from the device under normal operating conditions.
- Rated Capacity (kWh) The total amount of available energy within the storage system.
- Response time (Hz) The speed at which the storage device can respond to changes in the power system.
Links and Resources
- European Union Benchmarking Project on components for Renewable Energy Systems: www.benchmarking.eu.org
- RESDAS: Renewable Energy Systems Design Assistant for Storage: www.ecn.nl/resdas/
References
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