By Rikki Stancich in Paris

Back in 2008, Xcel Energy began testing a one-megawatt wind energy battery-storage system, using NGK Insulators Ltd.’s sodium- sulfur (NaS) battery technology. The objective was to demonstrate the system’s ability to store wind energy and move it to the electricity grid when needed, and to assess the viability of greater wind penetration on Xcel Energy’s system.

Earlier this month, Xcel Energy announced the preliminary test results of its wind-to-battery project. The bottom line?  It works.

The battery system has so far proven capable of shifting wind energy from off-peak to on-peak availability; of reducing the need to compensate for the variability and limited predictability of wind generation resources; of supporting the transmission grid system by providing voltage support, which contributes to system reliability; and of supporting the regional electricity market by responding to real-time imbalances between generation and load.  

Xcel Energy says that it opted for NaS technology on the basis of its high storage capacity and its ability to handle a large number of charge-recharge cycles (as would be incurred with an intermittent renewable energy resource). The storage technology is large scale and has potential for even larger scalability, while its dynamic response to system changes and its demonstrated commercial performance and availability lower the overall investment risk.

The battery comprises twenty 50-kilowatt modules, is roughly the size of two semi-trailers and weighs approximately 80 tonnes, and is able to store about 7.2 megawatt-hours of electricity, with a charge/discharge capacity of one megawatt. According to Xcel, fully charged, the battery could power 500 homes for over 7 hours.

WindEnergyUpdate speaks to Frank Novachek, Xcel’s director of corporate planning, to learn more about why ramp control may soon be indispensable; about the storage technology itself; and to gauge overall progress on wind energy storage.

WindEnergyUpdate: In the US, why is there a growing need for wind energy storage solutions?

Frank Novachek: We are currently the No. 1 wind provider in the US. As part of our environmental leadership strategy we are keeping wind in focus as a key renewable going forward. Due to our footprint, wind and solar are very strategic to us.

While we can manage now without storage, we believe that at some point, energy storage will be an appropriate solution. First, however, it needs to come down in price and improvements on performance need to be achieved before it is right for our customers.

Storage has an upside potential for competing other technologies, which is the basis for our current interest in storage, even if costs are not down yet. 

The work we are conducting now will enable us to evaluate when the appropriate time will be to use storage in lieu of other options.

WindEnergyUpdate: Xcel recently released the preliminary results from its wind-to-battery storage project in Minnesota.  Can you provide some insight into how the technology works?

Frank Novachek: The battery operates at 300 degrees Celsius and keeps the materials in a molten state to facilitate the transfer of various chemicals that make charging and discharging possible.

It is technically classified as a flow battery given the flow of materials from one side to another to allow the charge-discharge to occur.

The battery is hooked up to a power conversion system (PCS) to convert AC to DC for charging and to convert DC to AC for the discharge. This can occur based on the control signals provided to the PCS.  

Based on what we have seen, the switch from full DC to discharge can take place in a second. It is highly responsive. 

WindEnergyUpdate: How do these results compare with existing wind energy storage solutions?

Frank Novachek: From a performance perspective, the battery performed very well – even superior to other solutions available. But on a cost basis, we are not there yet.

Is it superior to other batteries out there? At the time we selected the technology, it was the only commercially viable battery system meeting our functional requirements that had a proven track record. Since then, there have been advances in other technologies that are now also commercially viable.

WindEnergyUpdate: Is Xcel Energy considering any other battery technologies?

Frank Novachek: Yes, but only from a demonstration perspective at this time.  We don’t see any other technologies that are cost effective enough to invest in yet. That is, to purchase for use in our operating systems. As a public utility, we are technology agnostic – we don’t generally invest in technology to develop it.

WindEnergyUpdate: How long will it be before wind storage becomes viable?

Frank Novachek: We don’t know yet – it is a question of reliability and cost. The cost is not yet at a point (based on the value streams we’ve studied) of deployment. It has to compete with more conventional means of integration, such as the gas turbines and combined cycle units we are currently using for wind integration.

WindEnergyUpdate: In order for wind energy to be considered 'dispatchable' how many hours of storage are required on average?

Frank Novachek: We are looking at how much storage as a function of output of the windfarm would be needed to control at different ramp rates.

If you have ramp rate control it gives you time to get conventional resources to catch up. Storage would help buy time for those resources to go up or down to deal with variability.

Over the next year, we will be working with the University of Minnesota to understand how much storage is needed to provide different levels of ramp rate control.

WindEnergyUpdate: How close is Xcel Energy to achieving this objective?

Frank Novachek: The studies being carried out by the University of Minnesota are scheduled for completion in July-August 2011.

They will also be looking at optimal options of placement of the battery. There are many schools of thought on where energy storage should be sited.

WindEnergyUpdate: How many batteries would be required for a 100MW wind farm, and what are the additional logistics and costs of incorporating an energy storage unit into a wind farm?

Frank Novachek: This ties into the ramp-rate control objectives. The issue is that you don’t know the ratio of storage to wind capacity.

More storage may be required as a function of total wind output for wind farms with fewer turbines; whereas a large wind farm may need fewer batteries compared to the total output due to the lower variability expected from larger wind farms.

If the wind resource is not good, you might need more storage than for a wind farm with a good wind resource.

A lot of different variables play into the equation.

WindEnergyUpdate: Realistically, how close is Xcel Energy to the end objective of
providing optimal wind power storage?

Frank Novachek: Closer than we were, but still a pretty long way off.

We still have a lot to learn before we can answer that question; the optimal point is a function of many parameters – there is not one single answer for each individual case. There will be unique design criteria for each application.

Yet we are very excited by the results. The project has gone very well and we have received a lot of interest.

WindEnergyUpdate: Who else is operating in this space at present?

Frank Novachek: There are several utilities looking into energy storage, including American Electric Power; PG&E in California; Public Service in New Mexico, just to name a few.

We are definitely not the only ones looking at energy storage; however, we are the first to look at energy storage in direct connection to a wind farm.

To respond to this article, please write to the editor:

 Rikki Stancich: rstancich@gmail.com