Back-up Storage: how much is it worth to utilities in the wind sector?

Can batteries handle decades of grid scale wind storage - or will new forms of storage be the future? And just how much is storage worth to utilities in the wind sector?

In an indication of the growing need for storage, as more wind power is added to the grid, utility-scale battery storage is now being tested in the Pacific Northwest, battery storage has just been compared with traditional technologies in a new Stanford study, and new compressed air energy storage (CAES) technologies take shape.

By Susan Kraemer

In Washington state in the US, the Pacific Northwest National Laboratory (PNNL) is testing battery technology at Nine Canyon Wind Project, a wind farm owned by Energy Northwest. Storage is becoming more crucial in the region, which serves as an example of what lies ahead as more renewables supply more power globally.

Renewables abound

In just ten years the very rainy Pacific Northwest, already blessed with abundant hydropower, has gone from very little wind to 6 gigawatts, and wind now provides between a quarter and a third of the total load. 

When seasonal snow melt boosts the region’s abundant hydro power, wind power has been curtailed. But the Northwest’s water power also supplies pumped hydro storage (PHS).

“We’ve relied on the advantage of a lot of hydro, to integrate intermittent resources like wind,” John Steigers, Generation Project Developer at Energy Northwest, tells WindEnergyUpdate.

He continues: “But that’s about maxed out, so going forward we’re going to have to deal with it a different way.”

There is the potential to add additional pumped storage, but not enough to store all the wind energy that has been curtailed. 

“If it were all built out, that would prevent only a fraction of the total wind power being curtailed,” he says.

Excess power: how to store it?

As PNNL’s partner, Energy Northwest will host a 500 kWh battery unit in a 20 foot container at its Nine Canyon Wind Project. Each module can store half a megawatt hour of energy.

In ‘real life’ the need would be for something more in the 20 MW plus range, but the PNNL scientists will ‘copy and paste’ duplicate copies of the one storage unit onto a virtual landscape of the region's entire energy supply and demand to model a full scale application. 

Among storage technology options, batteries are inherently modular and scalable, and portability is also attractive to Steigers, who sees the need for a boost in different parts of the grid at different times. 

For the Nine Canyon Wind Project, curtailments have been only for a few hours, and just in the spring when energy market prices are lowest, in spring.

And its wind farm losses from curtailment are also lower than most wind developers’, because as a publicly owned utility, there’s no tax liability, so it is not eligible for the Production Tax Credit, paid only on produced energy. 

But wind farm owners with PTC earnings suffer significant loss of income. “For the region as a whole the value of the curtailed wind is in the millions,” says Steigers.

What is storage worth and to whom?

Energy Northwest hopes to define storage applications of value to them not only as a wind farm owner, but also as a utility, and by extension of interest to their member public utilities, who are all very interested in storage. 

“Everybody in the industry looks at storage and says that has got to be worth something. But none of us have a good idea of really what is it worth to us,” he admits.

“Our member utilities get people pitching storage stuff to them all the time, and it’s hard for them to say do we want it and what’s it worth. That’s the big question we expect this to help solve.” 

But that is not the only question. Are batteries really suited for long term deployment?

Study compares batteries to CAES and PHS

The use of battery technology for bulk scale energy storage is fast evolving science, which is fortunate, because a study at Stanford’s Global Climate and Energy Project (GCEP) finds room for a great deal of improvement - in cycle life.

“What I was attempting to do with this study was just look at one small piece of that huge question,” co-author Charles Barnhart tells Wind Energy Update. “And that is what is the energetic performance of specific storage technologies in one specific storage application, and that is bulk storage supplementing renewable resources and returning electricity back to the grid.”

His study, published in the RSC journal Energy & Environmental Science, compared the various battery options with storing compressed air energy storage (CAES) in caves or in pumped hydroelectric storage (PHS) in terms of a new metric, Energy Stored On Investment (ESOI), to assess the relative energetic payback of each storage technology.

The amount of energy that can be stored by a technology, divided by the amount of energy required to build it is its ESOI, explains Barnhart. He notes that the higher the number the better. 

CAES and PHS benchmark

The study found PHS and CAES to both be better choices than other battery technologies for storage, which seems intuitively obvious, since caves and mountains and rivers already exist in nature. But storage in natural formations are limited by geography. Not everywhere is suitable, greatly limiting traditional storage options. 

But if it is better, just how much better? Over a 30-year timescale, for example, CAES turned out to deliver 240 times its energy cost compared with batteries that returned (at best; with Li-on) merely 10 times, and only 6,000 cycles compared with 25,000. 

However, co-author and GCEP director Sally Benson, calculated that batteries could make up the difference by extending their cycle life by a factor of between just three and ten.

“One of the main thrusts of the study was what can we do technologically, to increase the ESOI for electrochemical battery technology,” says Barnhart, whose research has now quantified just how much battery cycle life will need to be increased to compete. 

He referenced work being done by colleagues at Stanford who are engaged in research aimed at doing just that. “They are working on increasing cycle life to 40,000 cycles,” he says. “That’s more than enough to compete with CAES.” 

Innovating replacements for CAES and PHS

It is always going to be hard to compete with nature for sustainability. However, there is room for expansion of storage in semi-manmade CAES systems, not dependent on rare natural formations. Pike Research recently predicted that of the $122bn it expects to see invested in storage by 2021, most is going to be invested in new forms of CAES.

One 2 MW application of compressed air storage for wind farms was just commissioned in Texas in partnership with Conoco-Phillips. General Compression sees a future for storing wind in salt caverns, which are a relatively widespread natural formation.

SustainX is also innovating a closed system of CAES, but by storing the compressed air in large modular pipelines that can be buried a meter underground, it offers CAES that can be sited anywhere. 

New forms of pumped hydro essentially ditch the water and keep the gravity.

California’s ARESAmerica will use wind power to push heavy masses up to a slightly higher elevation on a closed-loop rail-track, and during descent, letting precisely controlled gravity drive turbine generators.

The company is working on offering scalable systems ranging from 100 MW with 200 megawatt hours of storage, to regional-scale storage systems with 16-24 gigawatt hours of energy storage capacity, and the ability to time-shift power delivery up to 8 hours.

What the key to this evolving innovation market will be is yet to be discovered. It is yet to be determined whether cycle-life is more important to stakeholders than capacity, portability, power or time-shift duration. 

But pilots like the PNNL test with Energy Northwest will find what is most important to utilities, and what storage is worth. And comparisons like the Stanford study establish benchmarks.

“The market doesn’t really exist yet, so the answers are not going to be traditional, they’re going to be new,” SustainX VP of Business Development, Richard Brody tells Wind Energy Update. 

“And the companies that are able to identify and innovate the places where storage can most effectively add value on the grid are going to be the ones that will be most successful,” says Brody.