The Holy Grail, the final piece in the renewable energy jigsaw, an unnecessary luxury - energy storage has been described as all of these in relation to large-scale renewables' penetration.

Storage that can be deployed at multi-MW scale is knocking on the door of the power industry and announcing its ability to, among other virtues, help deal with wind's intermittency, integrate renewables more smoothly into the grid, store renewable energy for sale at peak times and compensate for creaking power networks.

An array of technologies including compressed air, multiple species of batteries, flywheels and the use of molten salt with CSP plants may all have a part to play, and an old favourite, pumped hydropower, certainly will (see sidebars).

But what part, and to what extent, is still not clear. What is not in doubt is that storage is firmly on the agenda, especially in the US.

The US government has committed significant funds to help develop various storage technologies via its economic stimulus programme and Congress is looking at the possibility of offering incentives to grid-deployed storage.

In California, a bill working its way through the state legislature would require utilities to develop procurement programmes for energy storage systems to be achieved by the end of 2015, with an additional target at the end of 2020.

The California Energy Storage Alliance (CESA), an association of companies involved in developing storage technologies and systems, was predictably delighted when the state's assembly passed the legislation in June. "This landmark bill puts California at the forefront of a growing global market that will spur economic development," said CESA director Janice Lin.

Utility-scale Storage Set to Total 150 GW by 2015

That growth will be in the order of 22 GW of utility-scale energy storage worldwide by 2015, according to 'Renewable Energy Storage', a study by ABI Research released in May. ABI forecasts that by then, 150 GW of large-scale storage will be in place thanks to government incentives, increased performance through technical advances and declining costs of production.

Larry Fisher, research director at ABI, said he expects renewables to be the focus of much of the growth. "A good deal of utility-scale energy storage will be associated with renewable energy sources to compensate for their intermittency," said Fisher. "In the context of wind, we found Compressed Air Energy Storage well-suited for use on wind farms, as developments can be sited in places with appropriate geographic features to enable CAES. We forecast CAES will grow to nearly 1.6 GW globally by 2015."

The 290 MW Huntorf CAES station in Germany was commissioned in 1978.

While most of the noise surrounding storage is coming from the US, Fisher pointed out that the Asia-Pacific region, and particularly China, Japan and Korea, has seen heavy investment in battery technologies. "Those and other Asia-Pacific nations have aggressive renewable energy goals. Combined with government support, it appears likely the region will comprise the greatest share of global utility-scale energy storage, at least over the next five years."

Fisher expects batteries to play a key role at utility level, not least because of the huge amount of development going into systems for hybrid and electric vehicles. He also pointed to work on molten salt energy storage for solar thermal generation as a technology likely to see greater use in the future.

ABI's study is the latest to predict a significant role for bulk energy storage in helping to integrate renewables, and particularly wind power.

US Eyes Storage to Tackle its Transmission Issue

The advantage most often cited, as referred by Fisher, is storage's potential to compensate for intermittency in renewable generation levels.

According to Cian McLeavey-Reville, analyst at Delta Energy & Environment in Edinburgh, Scotland, a complete list of the potential benefits of storage could run to more than 100 items. Thus far, however, he believes much of the activity, particularly in North America, has boiled down to a single issue - compensating for the deficiencies in the transmission and distribution grid.

"In the US in particular they are worried about the integration of large, concentrated areas of wind production. It's not so much an intermittency issue as a transmission issue."

McLeavey-Reville said the combination of new large-scale power production in remote areas and an ageing grid will inevitably put strain on the system in specific areas. Storing what the grid cannot handle looks like an attractive option, at least in the short term, given the massive costs of upgrading the transmission system.

With the exception of pumped hydro developments in countries such as Spain and Scotland, storage has yet to gather the same pace in Europe as it has in Japan and the US.

European pioneers of renewables generation such as Denmark have tended to boast robust networks with the ability to handle what wind and other resources can throw at them.

"It's fair to say the storage industry has struggled slightly to find a foothold here [in Europe]," said McLeavey-Reville. "Any storage technologies that are being developed here are tending to focus on the North American market at the moment, because there's so much money floating around there. The fundamental drivers for storage just aren't as strong as in America."

That is not to say there is no European activity in the storage arena. In August, for example, the UK Energy Technologies Institute announced a technology demonstrator programme to develop a storage device capable of delivering a minimum of 500 kW on an 11 kV distribution network for around four hours. This would be enough to keep 400 homes powered for four hours, said the ETI, and would be used as a reserve to compensate for down-periods in renewable generation.

Delta Energy & Environment expects the continent's energy sector to take a much stronger interest in storage over the next few years. Indeed, it goes as far as predicting that what is currently a "very poor third behind Japan and the US" will become a "hotbed of storage activity".

The falling cost of storage technologies and the concentration of ever-larger renewable resources in far-flung areas of Europe are among the factors expected to drive activity.

But, while its momentum is undeniable, the case for bulk energy storage as a key imperative for renewables integration is far from universally accepted.

Wind Industry Fears Extra Costs of Storage 

There is unease in the wind power industry, for example, at the suggestion that wind inevitably needs storage to make it viable at high levels of market penetration, with the associated costs this would add.

An alternative school of thought sees storage as a "nice-to-have" rather than a "must-have", and points to other, cheaper weapons in the power system's arsenal such as more sophisticated use of flexible generators such as hydropower, better wind and solar forecasting, and greater use of demand response mechanisms.

The American Wind Energy Association (AWEA), for example, hailed the findings of a recent study by the US National Renewable Energy Laboratory (NREL) that looked at scenarios for integrating up to 35% renewables into the electricity network of the western US.

NREL's 'Western Wind and Solar Integration Study' concluded that 30% wind and 5% solar penetration was technically feasible and economic without the use of storage, provided other measures are put in place, including improvements to the transmission system. The cost of these improvements would be small compared to the overall benefits of the new wind power, AWEA claimed. While conceding potential long-term benefits of storage to complete power networks as costs come down, AWEA characterises it as "useful, but rarely essential" for wind at present.

On the other side of the coin, a study prepared for the California Energy Commission by KEMA points to the very immediate advantages of storing renewable energy, for example from its potential to cut reliance on conventional assets for balancing the system, contributing to an overall lower emissions power economy.

The debate will continue, but the money and political support flowing towards large-scale storage suggest its hour is at hand.

Andrew Lee is a freelance journalist and a former chief editor of Renewable Energy World.

Sidebar 1: Thinking Big - Compressed Air and Pumped Hydro Move in on New Ground

They say that everything comes back into fashion if you wait long enough, and compressed air energy storage (CAES) may be a case in point. Basic CAES technology uses the energy to be stored to drive air compressors. The air is stored underground until required and then released to drive a turbine that operates on less than 40% of the gas normally required due to its pre-compressed air input.

A commercial 290 MW CAES plant began operating in Huntorf, Germany, in 1978. A second was set up in Alabama in the US in 1991. And that appeared to be it, until the possibility of managing variable generation sparked new interest.

First Energy announced last November that it had acquired the rights to the long-mooted Norton Energy Storage Project in Ohio, based on a former limestone mine, and said the first phase could involve around 270 MW of generation capacity. With 9.6 million square metres of storage available, the company claimed the site has the potential to expand up to 2.7 GW.

Pacific Gas and Electric has received government match funding for a US$50 million demonstration CAES project in Kern County, California. The facility would be designed to store enough energy to deliver 300 MW for 10 hours. New York State Electric and Gas is also looking at a 150 MW demonstrator based in a salt cavern.

A striking direct link between renewables and CAES is the Iowa Stored Energy Park, which plans up to 150 MW of wind capacity with an underground storage facility.

The scale and siting issues of CAES projects make them slow burners, and the involvement of conventional generation in the process has led some to question their status in the renewables equation.

Nonetheless, while CAES promises much, as yet it has just a handful of ageing plants and some ambitious plans to its name. By way of contrast, pumped storage hydropower boasts around 127 GW of capacity worldwide and is growing at a rate that puts other storage technologies in the shade. China's state grid operator, for example, plans to raise its pumped storage capacity from 14 GW at the end of 2009 to 21 GW by 2015 and 41 GW by 2020, with the need to complement wind and solar generation given as the reason. Among a host of other developments so far in 2010, a 1.5 GW project has been announced in Vietnam, while Slovenia's first pumped storage facility started operation, and the first unit of the Dnister plant in Ukraine, one of the largest pumped storage plants in the world, came on stream.

While other technologies stake their claims and plug their potential, pumped hydro storage - just like hydro generation - is content to get on with adding capacity by the gigawatt.

Sidebar 2: Rapid Responders - Batteries and Flywheels

The battery - the storage technology of everyday life in its portable form - is set to play a role in the future of large-scale energy management, with two variants expected to feature significantly in the shorter term.

Japan has been the global pacesetter in large-scale battery storage for two decades, predominantly through the sodium sulphur (NaS) systems developed by NGK in conjunction with Tokyo Electric Power.

With more than 200 MW of installed capacity in the country, NaS is well-established in Japan, where it is used at substations and major industrial sites. Tokyo Electric Power believes NaS storage has a major part to play in the country's future renewables infrastructure, and has established a hybrid wind/NaS installation at Rokkasho in the north of Japan consisting of a 51 MW turbine array and a 34 MW NaS system.

NaS is also spreading its wings, notably through the recent installation of a 4 MW system at Presidio, Texas, the state's first utility-scale battery.

While NaS has established a track record at scale in Japan and elsewhere, market observers expect Lithium Ion (Li-ion) to quickly emerge as another serious contender.

A study by Pike Research in late 2009 forecast that Li-ion batteries will be the fastest-growing category for utility-scale applications, representing a 26% share of a $4.1 billion stationary energy storage market by 2018.

Many believe Li-ion's ability to offer scale applications will benefit from the considerable investment it is enjoying in the automotive sector as a key enabler of low-carbon vehicles.

"Utilities will be the downstream beneficiaries of innovation and investment in Li-ion batteries for the transportation sector," said Pike Research senior analyst David Link. "While Li-ion was once limited to consumer electronics devices, it is quickly becoming the battery of choice for electric vehicle manufacturers. Improved storage capacity and economics will lead the utility sector to adopt Li-ion as well."

Few companies have created as much of a stir in the storage market in recent years as A123 Systems, a Li-ion battery specialist that became something of a green-tech superstar when it floated last year.

A123 employs electrode technology first developed at the Massachusetts Institute of Technology in battery systems for a range of markets, with grid applications high on its agenda. In August, 2010 the company signed a deal with AES Energy Storage to supply 44 MW of grid-scale systems. A123 has also worked with AES on a 12 MW frequency regulation project in Chile, said to be the country's first energy storage facility.

Alongside NaS and Li-ion, work is underway on flow battery technologies, where an electrolyte flows through an electrochemical cell. The technology is viewed as one of the most promising options thanks to its promise of fast response and long life.

The development of commercial flow battery products has been relatively slow. However, systems such as the cellcube range from Austrian group Cellstrom, which made its debut earlier this year at Intersolar Europe 2010, are now appearing on the market.

The cellcube vanadium redox flow battery

 

The cellcube is a vanadium redox flow battery, employing vanadium salts in its electrolyte. According to Cellstrom, the use of vanadium results in a "virtually unlimited" number of charge/discharge cycles and a 20-year lifespan.

The larger of the two cellcubes features an output of up to 200 kW and a maximum storage capacity of 400 kWh. Cellstrom is aiming the turnkey battery system at applications such as renewable energy storage, back-up power supplies and load levelling and balancing.

Frequency regulation is firmly on the radar of those developing flywheel storage systems - 'kinetic batteries' that store energy in a high-speed rotating matrix and then discharge it as electricity when required. Flywheels are hailed by their supporters as the 'greenest' storage solution as they do not consume fuel or produce emissions.

Already used for applications such as UPS, the flywheel now has bigger ambitions at utility level.

In August, Beacon Power Corporation closed the financing on its flagship 20 MW flywheel energy storage plant in Stephentown, New York, thanks to a $43 million loan guarantee from the US Department of Energy (DoE).

The Stephentown project, billed as the first utlity-scale plant of its kind in the world, is designed to aid regulation of the power grid, enabling greater use of renewable resources and reducing dependence on conventional assets. Operational by the end of the year, it will eventually provide around 10% of New York's daily frequency regulation capacity, according to Beacon Power.

In a move to directly ally flywheel and wind, Beacon has also installed a system at a wind farm in Tehachapi, California, as part of a demonstrator project for the California Energy Commission.

The company says it is now developing the next generation of its flywheel technology, aimed at storing four times the energy at one-eighth the cost of its current flagship Smart Energy 25 system.