Energy Storage - Supporting Greater Wind Energy Usage
12.16.05 Richard Baxter, Sr. Technology Analyst, Ardour Capital Investments, LLC
Coupling energy storage technologies with wind turbines can solve many of wind power?s operational issues and support the continued expansion of wind energy production. It should be noted that many types of renewable energy production already benefits from energy storage technologies. By decoupling the production and delivery of energy from renewable resources, storage technologies can make the generated energy more useful and more valuable.
To date, the wind power industry has made great strides in enhancing the capability of wind turbines and how they are integrated into the overall power market. Although the direct production cost may now be competitive with other power generation resource at certain locations, its effective usage cost is sometimes still higher due to inherent qualities of the wind resource. Storage technologies can provide additional flexibility to mitigate these issues.
Small Grids: Provide system stability (frequency and voltage).
Large Grids: Provide local system stability and enhance transmission deliverability.
Storage Technologies
A number of energy storage technologies are currently in use or being evaluated for use in conjunction with renewable energy resources. Some of these technologies include:
Flywheels: Flywheels store energy in a rotating mass of either steel of composite material. Through the use of a motor/generator, energy can be cycled (absorbed and then discharged) a great many times without reducing the life-span of the device. By increasing the surface speed of the flywheel, the energy storage capacity (kWh) of the unit can be increased; by increasing the size of the motor/generator, the power (kW) of the unit can be increased.
Flow Batteries: Flow batteries store energy in charged electrolytes and utilize proton exchange membranes similar to fuel cells. By flowing the (charged or uncharged) electrolytes through the cell, energy can be cycled through the unit. By adding additional electrolyte, the energy storage capacity (kWh) of the unit can be increased; by increasing the number of cells, the power (kW) of the unit can be increased.
Compressed Air Energy Storage (CAES): CAES facilities store energy in compressed air held in underground chambers. These facilities charge (compress the air) the cavern at night with low cost system power; this air is then used as input for a gas turbine during peak price periods during the day, allowing all of the energy output to generate energy instead of compressing air in pre-combustion. By increasing the volume of air in the underground chamber, the energy storage capacity (kWh) of the unit can be increased; by increasing size of the compressor and turbine, the power (kW) of the unit can be increased.
Applications
Remote Power ? Island Grids
Small, remote power grids, many times referred to as (or actually exist as) island grids rely heavily on diesel reciprocating engines for their power. Although reliable, these units must respond to significant changes in daily or hourly load, with peak power levels many times far above average load levels. For these reasons operating costs on these systems can be extremely high due to transportation cost of the fuel and mandatory minimum run-times of the diesel engines. In many locations, wind turbines are being added to compliment and hopefully supplement these power sources. To assist this wind energy to integrate further and in a more meaningful way, many developers are looking to energy storage facilities to balance out the constantly changing power supply and demand levels into a far more effective operating regime.
The benefits of using wind energy can be quite high. A number of studies by US Government Laboratories (NREL, LLNL, etc.) have shown that adding wind to a diesel-powered local grid can reduce fuel consumption by 40%-50% and total costs by 30% to 50% for areas with plentiful wind resources.(1) However, because of the small size of these power grids (lack of system inertia, etc.) simply adding wind turbines to small power grids cannot be done haphazardly?a systematic review of the load and potential additional wind turbines must be undertaken to ascertain potential benefits, and to determine what level of wind penetration is best. For many of these power grids, the opportunity exists to have wind resources well in excess of 50% of the peak load.
The same studies that showed that increasing the wind penetration can lower the diesel fuel costs on these systems also showed that adding a storage component can gain an additional 10%-20% in system cost reductions. Although wind turbines provide power with no fuel cost, they bring with them operational characteristics that cause the overall system to operate at sub-optimal conditions many times due to the variability of the wind energy, the non-dispatchability of the wind energy, and the additional system stabilization requirements (frequency and voltage) required. By alleviating some of the stress on the system by operating as a dynamic source and sink for power (a shock absorber), energy storage can be a beneficial additional to these island grids for three general reasons: reducing diesel starts/runtime, providing system stability, and improving the reliability of supply from increasing the level of wind penetration for the system.
The value of energy storage to the system increases as the wind penetration increases, as there will be an increasing amount of time that the available wind power exceeds the total system loads. According to one NREL study(2), at 50% wind penetration, storage can provide 20% greater fuel saving and 20% fewer diesel run-tine than non-storage wind/diesel systems alone.
Example?Dogo Island, Japan(3)
The installation on Dogo Island, a small island just off the coast of Japan, is an example of how a flywheel energy storage system can provide the stabilizing capability lacking on many island power grids. In 2003, Fuji Electric installed a 200-kW UPT KESS from Urenco Power Technology in conjunction with a 3x600kW installation of De-Wind D4 wind turbines to evaluate how wind generators can be a viable source of power on remote islands with weak links to the mainland power grid by smoothing their irregular power output. Through incorporating the flywheel-based energy storage unit into the installation of the wind turbines, Fuji Electric sought four goals: to stabilize the frequency variations stemming from the turbines, to capture excess energy from short-term wind gusts, to optimize the operation of (or eliminate the need for) diesel generators on the island, and eliminate the need for additional spinning reserve due to the introduction of the wind turbines.
Results to date have been promising; by acting as both a dynamic sink and source of energy, the UPT KESS improved the island?s power grid efficiency and increased the penetration rate of the wind turbines. The flywheel unit?s ability to provide a stabilizing capability to the highly variable wind turbine power was found to be essential in allowing Fuji Electric to connect the wind turbines to the island's relatively weak electrical transmission system. Because of this successful outcome, Fuji Electric is now looking for further deployment opportunities of the UPT KESS technology to provide reliable wind-generated energy as a viable supply alternative in other locations.
Example?King Island, Australia(4)
King Island, located off the Australian coast has been installing wind turbines to complement and hopefully supplement the existing four 1.5-MW diesel generators. However, by the time the fifth wind turbine was installed (the wind turbines ranged in size from 250-kW to 850-kW) the balancing of the island?s grid was becoming problematic. To assist with system balancing, the local utility?Hydro Tasmania?subsequently installed a 200-kW/800-kWh VRB-ESS flow battery system from Pinnacle VRB Ltd., (a subsidiary of VRB Power Systems at the time).The VRB-ESS has provided three benefits to the power grid since its start-up in November of 2003: load-shifting off-peak wind-energy to on-peak demand, improved the operation of diesel units (reduce frequent startups with minimal run-time), and provided frequency regulation and voltage control to assist with higher wind energy integration.
Capacity Firming
If energy storage technologies are to play a significant role in conjunction with wind power in general, it will be through firming the delivery of wind power from large grid-integrated wind farms. Rather than cycling all of the output from the wind turbines through the storage facility, the capacity firming strategy focuses on providing sufficient support to the output of the wind-farm to ensure a guaranteed minimum of on-peak energy sales to reduce additional ancillary service requirements or energy reserves support and thus improve the total economics of the wind farm. Determining the needed power rating for energy storage unit to support a wind farm for this strategy requires knowing and understanding such issues as the size of the wind-farm, the variability of the local wind resource, the local power transmission capability, and the average load profile of that local power system. Many times this can require sizing the power rating of the unit at only 20% (or less, depending upon the strategy followed) of the size of the wind-farm.
More important to the strategy of the ensuing combined wind/storage combination is the determination of the how much energy storage (MWh) is required. For areas of constant wind-speed variability, high cycling energy storage facilities with only a small storage capacity may be useful?akin to the frequency regulation service provided on small island grids. By constantly absorbing and discharging any excess wind energy, you can improve the delivery and provide a more stable power output from the facility, benefiting the power flow on the grid; this would be of especially benefit in areas or weak transmission systems.
On a more common basis and for larger wind projects, wind energy could be stored during off-peak periods or during any time when the transmission of the output is constrained. This energy could then be delivered later to supplement existing wind generation during on-peak periods to ensure a minimum (for contractual delivery) but hopefully a maximum (for greater profit) energy sales, depending upon the transmission availability at the time.
Example?McCamey, TX(5)
McCamey, TX remains one of the State?s centers for much wind power development activity due to its preferential wind resource potential in the area. Unfortunately because of its remoteness in West Texas, developers can easily build out more wind generation than the transmission grid can easily handle, causing congestion problems. Although plans exist for additional transmission upgrades, new wind farm development is expected to match or even surpass these upgrades for many years to come. To alleviate this near-term transmission constraint problem and provide room for additional wind generation in the area, the Texas State Energy Conservation Office (SECO) commissioned a study (led by the Colorado River Authority) to determine what benefits a large-scale energy storage facility would have for transmission. To support this expected continued mismatch between wind power and transmission capacity, SECO chose a Compressed Air Energy Storage (CAES) facility with 400-MW compression, 270-MW generation, and an extremely large 10,000 MWh storage capacity (at full power, 25 hours capacity for compression / 37 hour capacity for generation). The facility would be used to store power during periods when congestion on the transmission lines constrained-off the growing wind resources.
Unfortunately, because of the wind pattern in the area?there are times when the wind blows strongly and continuously for days at a time, the modeling of the project showed that the CAES facility could become full, and the CAES plant becomes unavailable to provide additional energy storage. Finding an alternative to an additional transmission construction was the single specified desire for the study. Therefore, it was found that the CAES facility could not precluding the facility from alleviating 100% of the transmission constraint, and thus it did not substitute for transmission lines on a in this instance.
Extending the evaluation of the CAES facility past purely transmission replacement role, however, it was found that there are a number of benefits that the facility could provide. First, the CAES facility would allow more wind generation (up to 400-MW) to be built out in the area with only minimal curtailment, and would provide significantly better capacity firming of the wind farms for the area?allowing and providing assurance for far more power to be delivered during peak demand periods. Combining wind with storage also ensures that wind can claim credit for operating reserves (equal to the amount of CAES generation). Although this was not a significant payment, having this capability does add to the total value of the facility (the value of any facilities stems from not just one revenue stream, but many), and it provides additional firm capacity for the system operator to call upon, something expected to be needed in the near-future especially as the amount of wind generation continues to grow.
By operating the CAES facility to support wind generation in the area, wind energy curtailment reduction totaling over 600 GWh annually was achieved (compared to the area without the storage facility) in the modeling study, and provided several million dollars profit annually above and beyond what would be required for a positive return on the CAES facility investment. Because of this outcome, work continues on siting a CAES facility for this role in the area.
References
(1) Remote Power Systems with Advanced Storage Technologies for Remote Alaskan Villages, Isherwood, W., Smith, R., Aceves, S., Berry, G, Clark, W., Johnson, R., Das, D., Goering, D., and Seifert, R., December 1997. (UCRL-ID-129289)
(2) An Analysis of the Performance Benefits of Short Term Energy Storage in Wind Diesel Hybrid Power Systems, Shirazi, Mariko, and Drouilhet, Stephen, ASME Wind Energy Symposium, 1997. (NREL/CP-440-22108)
(3)Urenco Power Technology website, www.uptenergy.com
(4) The Multiple Benefits of Integrating the VRB-ESS with Wind Energy Producers?A Case Study in MWH Applications, Hennessey, Timothy D.J., AWEA Conference 2004.
(5) Study of Electric Transmission in Conjunction with Energy Storage Technology, Lower Colorado River Authority, August 2003.
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