Environmental Engineering Reference
In-Depth Information
Case Study 2: Xcel Energy to Test Storage of Wind
Power Using 1-MW Battery System
Xcel Energy soon will begin testing a cutting-edge technology to store wind
energy in batteries. It will represent the first use of the technology in the
United States for direct wind energy storage. Integrating variable wind and
solar power production with the needs of a power grid is an ongoing con-
cern for the utility industry. Xcel Energy will begin testing a 1-MW battery
storage technology to determine its ability to store wind energy and move it
to the electricity grid when needed. When fully charged, the battery could
power 500 homes for over 7 hours. Xcel Energy signed a contract to purchase
a battery from NGK Insulators Ltd. that will be an integral part of the proj-
ect. NaS batteries are commercially available and versions of this technology
are already in use in Japan and in a few United States applications, but Xcel's
installation will be the first United States application of the battery as a direct
wind energy storage device. The 20- to 50-kW battery modules will be roughly
the size of two semi trailers and weigh approximately 80 tons. They will be
able to store about 7.2 MWh of electricity, with a charge-discharge capacity
of 1 MW. When the wind blows, the batteries are charged. When the wind
calms, the batteries supplement the power flow. Partners in the project with
Xcel Energy include the University of Minnesota, the National Renewable
Energy Laboratory, the Great Plains Institute, and Minwind Energy LLC.
Xcel is testing emerging technology and energy storage devices as part of its
overall “smart grid” strategy intended to modernize and upgrade its grid to
allow easier integration of renewable energy sources.
Vanadium Redox Batteries
The technical boundary conditions to electricity storage results from two dis-
tinctly different requirements based on the service conditions of the battery:
(1) load leveling and peak shaving and (2) seasonal energy storage. For load
leveling applications, the storage medium must have a high power density,
be able to recharge at high rates for many cycles, and be able to withstand
deep discharges. For seasonal storage, a battery must have large capacity,
low self-discharge rate, and be capable of operating on a great number of
shallow cycles. The current trend in wind energy is to install turbines in a
large wind farm connected to a single point in the grid.
Due to wind power fluctuations and limited wind predictability, the ideal
energy storage device for wind farms would achieve load leveling and sta-
bilization of turbine output. The storage device would store power at night
when the wind blows hardest and electricity usage and rates are low, then
deliver power to the grid during the day when transmission lines reach
capacity, and thus allow a utility to profit from higher daytime rates. To
attain a higher degree of controllability, an energy storage system must have
the capacity to store a lot of energy for short periods and deliver it during
