Wind Power-to-Battery Project

As the nation’s number one wind power provider and number five solar energy provider, Xcel Energy has demonstrated its renewable energy leadership in the utility industry. Xcel Energy continues to aggressively pursue wind and other types of renewable generation technologies in line with a strategic vision of a clean energy future.

However, as more wind capacity comes online and meets a greater amount of customers’ load obligation, system impacts will become consequential and have to be addressed. With a large penetration of wind already in the Xcel Energy balancing footprint and plans to add more in the near future, Xcel Energy is seeking innovative ways to integrate renewable energy. One potential solution is large-scale electrochemical energy storage.

Xcel Energy is conducting the Wind-to-Battery (W2B) Project to evaluate the overall effectiveness of sodium sulfur (NaS) battery technology in regards to its ability to facilitate the integration of wind energy onto the grid. As part of this demonstration project, Xcel Energy is investigating the ability of the technology to provide system benefits, the cost-effectiveness of the storage device, and methods and procedures to evaluate other types of energy storage technologies in the future.

Through this small-scale demonstration project, Xcel Energy can evaluate energy storage technology at a modest level of investment and customer impact. By doing so, the company will promote the future deployment of only proven technologies that meet or exceed cost, reliability, and environmental requirements.

This report summarizes the primary testing phase of the project, which consisted of a technical evaluation of the battery-based Distributed Energy Storage System (DESS) and grid-related performance data under multiple modes of operation. Project analysts have completed initial testing of the DESS for all modes of operation and collected system performance data accordingly.

From preliminary analyses of the data, Xcel Energy has assessed the effectiveness of the technology for each mode of operation and gained a better understanding of the general operating characteristics of the technology.

Overall, the battery met expectations by performing successfully in all modes tested. However, project analysts still recommend additional testing to better understand the capabilities and limitations of the storage device.

Basic Generation Storage (Time Shifting) was tested by scheduling the battery to discharge during defined on-peak periods and charge during defined off-peak periods at a rate that was proportional and coincident with the power output from the wind farm. Overall, the DESS performed as expected for the majority of scenarios tested for both the wind-only and wind-grid charging variations.

Project analysts did find that a modification to the Power Conversion System (PCS) software is required for one of the discharge profiles. During the testing period, the 1 MW wind farm scenario was incapable of fully charging the battery during the allowed charging ‘window’ of 8.5 hours, while the 10 MW wind farm generated more wind energy than needed. Project analysts recommend additional testing, especially at and around the 5 MW scenario, to better understand the optimal ratio of wind farm capacity to DESS capacity for time shifting applications.

Economic Dispatch was tested whereby the DESS followed set-points based on an algorithm that uses forward and spot energy prices in the MISO market along with settings from the user.

Although the battery was never officially offered into the MISO market, project analysts estimated the settlements results. The DESS performed as expected in this mode of operation.

Although the arbitrage potential was limited due to market conditions, project analysts feel that improved results are possible by optimizing the control algorithms. Moreover, project analysts also recommend performing additional testing using different market nodes and pricing information from previous years to better estimate potential financial returns over an extended period of time at various physical locations.

Frequency Regulation was tested whereby the DESS followed a frequency regulation signal derived from changes in the Area Control Error (ACE) for the MISO market. Even with the frequent temporary system alarms, the DESS performed well in this mode. On a continuous basis, the device followed the rapidly changing set-points issued by the NSP Energy Management System (EMS) in a timely and accurate manner and displayed excellent ramping capabilities. Project analysts recommend additional testing for this mode over time to determine if any long-term damage is incurred by the batteries as a result of the rapid, frequent charging and discharging of the battery.

Wind Smoothing (Ramp Rate Control) was tested by using a 1st order lag function to vary the charging and discharging rates of the battery based on the output of the effective wind farm. The test results were mixed due to range limits in the PCS source code, and additional testing will be required once fixed. The tests that produced valid data indicate that the DESS is able to effectively limit the rate of change in the 1 MW wind farm scenario, but the rate of change was too great for the 1 MW DESS to handle in the 10 MW wind farm scenario.

Additional testing is recommended to better understand the optimal ratio of wind farm capacity to DESS capacity for wind smoothing applications, especially at and around the 5 MW scenario. Wind Leveling (Steady Output Control) was tested by varying the DESS charging and discharging operations to minimize the difference between the expected and actual power output for the wind facility. Overall, the DESS performed well for the 1 MW and 5 MW scenarios.

The results for the 10 MW scenario varied too much, preventing analysts from drawing any detailed insights. For all the scenarios, the DESS performed as expected, responding to changes in the output from the effective wind farm rapidly and accurately. Additional testing data is needed for the 5 MW and 10 MW scenarios to better determine the leveling capability of the DESS for a more statistically valid set of wind profiles.

In addition, project analysts recommend testing this mode using forecasted values from the company’s various wind forecasting programs and then comparing the results to the results obtained using the persistence methodology. This will enable Xcel Energy to identify any potential synergistic benefits available when integrating wind energy by incorporating new technology into the company’s daily business operations.

This report also contains a discussion of policy issues identified by a work group convened by the Great Plains Institute. Also included is a comprehensive technology overview for the NaS Battery, the PCS, and the overall DESS System Architecture, which may be useful in understanding the testing process and results.

The next and final phase of the project will be to use existing and future system performance data to complete the evaluation of all the identified value propositions contained in the project objectives. Once complete, project participants will be able to determine the ability of the DESS to facilitate the integration of larger penetrations of wind energy on the grid and assess the cost effectiveness of the technology. This will be accomplished with the assistance of the University of Minnesota and the National Renewable Energy Laboratory, whose findings will be submitted June 2011 and summarized into a final report (Milestone #6) for the Renewable Development Fund in August 2011.

Project Description

A 1 MW, 7.2 MWh NaS battery purchased from NGK Insulators Ltd. (NGK), a Japanese firm involved in the manufacture and sale of power-related equipment, was installed near the 11.5 MW Minwind Energy LLC (MWD) wind facility in Luverne, MN. The battery is located at the newly constructed “W2B Substation” adjacent to both Xcel Energy’s existing Rock County (RCY) Substation and MWD’s substation.

Next to the battery, S&C Electric (S&C), a Chicago-based company that provides equipment and services for electric power systems, designed, built and installed a stand-alone power conversion system (PCS), which includes a local monitoring, data collection, control, and communication system. Herein, the combined NaS-PCS system is referred to as the Distributed Energy Storage System (DESS). S&C also installed a 175 kW backup generator at the NaS Substation to provide backup power for the battery heaters in the event that grid power was lost at the site.

GridPoint Inc., a firm involved in smart grid technology, provided a remote two-way communications and control system for system integration, remote monitoring and control, and data access. GridPoint’s system enabled the battery to respond to Automatic Generation Control (AGC) and market-driven control set-points.

Project participants selected the NaS technology for multiple reasons. The battery has a high energy storage capacity, can handle a large number of charge-discharge cycles, is capable of dynamic operation, and has demonstrated commercial performance and availability. In addition, the technology is capable of large scale deployment in the future.

Xcel Energy selected the MWD facility for several reasons. First, the company thought it was important to locate the battery next to a wind farm to avoid any potential latency issues when trying to relay output data from the turbines to battery when operating in a “wind-coupled” mode of operation. Furthermore, the company wanted a wind farm with an installed capacity in the range of 10 MW so as not to overwhelm the 1 MW battery. Also, because Xcel Energy owns a substation at the site, the project team was able to minimize land purchase/usage fees.

Finally, expressing interest to participate in the project as a partner, MWD offered the use of its interconnection transformer to the transmission system. By using the MWD transformer, Xcel Energy avoided the need to purchase and register an additional transformer.

Project Objectives

Along with external project partners, Xcel Energy is conducting its W2B Project to evaluate the ability of utility-grade, large-scale electrochemical storage to provide system benefits specific to wind (from the perspective of both a wind farm owner and a balancing authority) and the bulk electric grid in general.

The objectives for the W2B Project are the following:
 Evaluate the ability of large-scale battery storage technology to effectively shift wind energy from off-peak to on-peak availability;
 Evaluate the ability of the DESS system to reduce the need for the utility to compensate for the variability and uncertainty impacts of wind against other grid balancing procedures;
 Evaluate the potential for battery-storage technology to provide ancillary service support to the grid;
 Assess the obtainable value of storage in the Midwest ISO (MISO) market for current wind penetration scenarios; and
 Assess the overall operating characteristics of the DESS system, including impacts on system performance as a function of operational mode and external weather conditions.

Modes of Operation

To meet these objectives, the W2B project analysis team evaluated the DESS under multiple modes of operation (see table below) to obtain a thorough understanding of the system’s range of capabilities.