Full charge ahead

Compressed air energy storage (CAES) is not a well-known technology, yet it has been around for years and, as E&T discovers, is enjoying something of a revival.

A power supply cooperative in Alabama, USA, may seem like an unlikely host for the commercial demonstration of a novel, utility-scale energy storage technology, but for the past 18 years PowerSouth Energy Cooperative has successfully been operating one of only two compressed air energy storage (CAES) facilities in the world.

One full charge from the 110MW McIntosh CAES plant can provide enough electricity to meet the demand of 11,000 homes in the southern state for 26 hours. But, perhaps more importantly, it provides PowerSouth with a useful tool to manage the resources of its system in an effective and economic manner.

"We are a power supply cooperative with a small grid and around 2,000MW of capacity," explains McIntosh plant manager Lee Davis. "But our customer base is primarily residential and we experience high peaks in demand. We chose CAES because it allows us to store energy from our conventional plants when electricity demand is low, and to generate electricity when demand is high."

It is the ability of the CAES plant to optimise the utility's resources that is attracting renewed interest in the technology. There is recognition that the challenges faced by PowerSouth in meeting large swings in electricity demand mirror the difficulties that other, larger utilities now face due to the rapid increase in renewable generating capacity on the grid.

Renewable bottlenecks

In Europe and North America in particular, the recent growth in renewable energy is creating technical challenges for generators and transmission system operators alike. Most renewable energy technologies are not demand-driven and generate electricity in an intermittent fashion, and conventional generating units on the grid must compensate accordingly.

In addition, the strong level of political support for renewable energy technologies has caused a boom in particular 'hotspots', resulting in transmission bottlenecks.

These challenges all point to a need for increased levels of energy storage, argues Georgianne Peek of the Sandia National Laboratory in New Mexico. "Wind is a wonderful technology but some utilities have so much on their system that their conventional plants cannot handle the ramp up or down as the wind levels fall and rise," explains Peek. "Transmission systems are ageing and, although there is lots of new build, right of way is an issue. Storage in strategic locations can help to defer new build."

In fact, the need for energy storage on the grid in the US is becoming a pressing issue. In July 2008 the US Department of Energy (DOE) published a report in which it noted that wind energy could account for 20 per cent of electricity generation by 2030. Last year the country overtook Germany to become the world leader in wind generation, with over 21,000MW of capacity.

Coupled with this is the fact that the electric power industry in the US runs at very low capacity factors - as low as 40 per cent - according to the Electricity Advisory Committee (EAC), a panel of experts that advises the US government on industry issues. Increased levels of renewable energy on the grid will lead to even lower capacity factors for traditional generation sources.

Bulk storage

Utilities looking for large-scale energy storage solutions have traditionally turned to pumped storage hydropower. However, such plants are only economic on a large scale and take many years to construct. Available sites for pumped hydro plants are dwindling, both in the US and Europe.

CAES technology is the only viable bulk storage alternative to pumped hydro. A CAES plant uses off-peak electricity to drive motors that compress air into an underground reservoir. During times of peak demand, the compressed air is withdrawn, heated, and passed through an expansion turbine to drive a generator. The world's first commercial CAES plant was commissioned in 1978 and is located in Huntdorf, Germany.

The CAES facilities at Huntdorf and McIntosh use engineered salt caverns as the storage reservoir for the compressed air, although other types of geological formation are thought to be suitable, including sandstone aquifers and depleted natural gas fields. EPRI studies show that more than half of the US has geological conditions that are potentially suitable for CAES.

While the use of salt domes has been proven at both existing CAES plants, the engineering involved in preparing the cavern can take up to two years. In Ohio, Norton Energy Storage, a firm backed by private equity group Haddington Ventures, is proposing to use a limestone mine for a 2700MW CAES development. Although tests carried out in 2000 on the mine's rock formations indicate that it would be well suited to air storage, the project is still under development.

Aquifer storage

One project that appears to be advancing is the Iowa Stored Energy Park (ISEP), which is being developed by a group of municipal utilities with the backing of DOE funds through Sandia National Laboratories. This project will be the first CAES plant to use an aquifer as the storage reservoir.

"The cavern is a sandstone aquifer with a tight shale caprock," says Kent Holst, development director with the Iowa Stored Energy Plant Agency (ISEPA). "We have done some very extensive seismic work to determine the size and configuration of the formation and will soon begin drilling test wells as well as testing migration rates of water and air injection."

The developers of ISEP are also benefiting from data from a nearby sister formation that is used as a natural gas storage reservoir. Core samples will be drilled in 2009 and sent to Sandia for analysis, and Holst hopes that 2010 will see the start of specific design work on the project.

"We are confident about the Iowa site," notes Peek of the Sandia National Laboratory, which is providing ISEPA with geological expertise and technical project management. "The core samples will give us more information on the permeability and attributes of the formation and the caprock, and how the structure will stand up."

Like other utilities, those in Iowa are turning to CAES technology to overcome the problems created by the projected large levels of wind capacity. According to Holst, by 2013 as much as 30 per cent of the load in Iowa could be supplied by wind. "This goes beyond the point where wind creates problems on the grid, and so was a major factor in the selection of CAES."

Second generation

The potential for CAES to relieve utilities of the headaches created by wind generation is something that US utility PSEG is working hard to promote. In August 2008 the company teamed up with energy storage pioneer Dr Michael Nakhamkin to create Energy Storage and Power LLC (ES&P), a joint venture tasked with marketing and developing second-generation CAES plants.

"Dr Nakhamkin has been active in CAES since the early 1980s and the McIntosh unit was developed under his patent," explains Roy Daniel, CEO of ES&P. "However it is a very customised plant with all the equipment on a single shaft and is complicated to operate."

Nakhamkin has been working with EPRI in the US to bring the technology forward, making it more efficient and simpler to operate than the McIntosh plant. The result is second-generation CAES technology, for which ES&P owns the patent.

"Second-generation CAES represents a huge advance over the technology at McIntosh," notes Daniel. "It uses standard components - including the compressor and gas turbine - and is totally scalable so that plant size is flexible according to geology."

ES&P's main focus is now the development of a reference design plant. Interest in the technology is building, according to Daniel, and EPRI has instigated a plan to sponsor two CAES plants - one of 15MW and another of 350MW.

"There is huge potential for this technology," concludes Daniel. "We call it a 'greening' technology, because it amplifies existing renewable energy units and allows optimisation of the whole system."

Peek agrees, and believes that CAES plants should also be eligible for carbon credits. "Even if a CAES plant is using electricity from conventional plants to compress air, it can reduce emissions of greenhouse gases because it enables all power plants in a system to run at their optimal set point."

Advanced technology

Another benefit of CAES technology is that it allows utilities to buy low-cost, off-peak energy and sell it during peak periods. According to Roland Marquardt, research and development engineer with Germany's RWE Power, opportunities to exploit this price spread will grow.

"Europe is poised for a large expansion of offshore wind capacity, which will lead to price volatility because generation from these plants is dependent on the wind rather than on electricity demand," says Marquardt.

Together with GE, RWE is pioneering development of advanced adiabatic CAES technology (AA-CAES), which captures heat from the compressed air to make the cycle more efficient. The two companies in 2007 signed an agreement to investigate the feasibility of the technology and ways of overcoming the technical challenges.

"In a conventional CAES plant, heat generated during compression is rejected by intercoolers and cool air is stored in the cavern," explains Marquardt. "When the plant is generating electricity the air must be heated up by burning natural gas."

He continues: "With AA-CAES, we get around this heat rejection by using minimal intercooling during the compression phase and capturing the heat in a thermal energy storage system. The compressed air flows over a solid material - such as ceramic bricks - so that the heat can be transferred, and when the air is discharged from the cavern you let it flow back over the material to pre-heat it before it passes to the expander."

Development of AA-CAES is not without its challenges, however. According to a recent study carried out by the European Commission, AA-CAES systems will not be able to use standard turbomachinery and the development of completely new equipment will be required.

"The main challenge is the compressor outlet temperature, which reaches around 620°C," says Marquardt. "Although the European study suggested development of a completely new machine, our feasibility study has investigated the possibility of developing the compressor section using mainly standard components. We have looked at a lot of options and have narrowed it down to two or three that could be realised with existing GE equipment.

"We are confident that technical solutions are available to overcome these challenges because in gas turbine technology temperatures of this kind are not really an issue. We also think that the air expansion turbine will not be a big challenge."

RWE and GE are now in the final stages of their feasibility study, and hope to put together a consortium for development of a demonstration project. While Marquardt is guarded about the costs, he is clear that the market conditions have to be right for this technology to go forward.

"This technology becomes economically feasible when the spread between peak and off-peak is large, for example when there is a lot of wind on the grid," says Marquardt.

Barriers

But in spite of all the apparent benefits of CAES technology, its uptake may be hindered by a number of barriers, including a lack of regulatory history to guide regulators and investors. According to the EAC, there is no overall strategy in the US on the incorporation of energy storage technologies into the power grid. This leaves utilities uncertain as to how investments in energy storage should be treated and how costs could be recovered.

Another issue is the fact that CAES benefits both generation and transmission companies, and this leads to confusion about how energy storage should be regulated.

"In the US there is separation of transmission and generation," says Peek. "CAES can benefit both but neither is willing to invest. We need a better policy framework."

ES&P's Daniel agrees, and says that the technology should be given more support financially - as renewable energy technologies are - in order to encourage the construction of CAES plant. "We need public policy changes and support mechanisms to get the first wave of plants out there," says Daniel. "This will provide banks, utilities and developers with the experience they need, and will also help to prove the technology and bring down the price."

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