Why reinvent the wheel with energy storage?

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By Alex Nicolson

Despite rapid expansion, global electricity storage represents less than 2% of the world’s electric power production capacity according to the Center for Climate and Energy Solutions. In response, subsidies, policy mandates and innovation are being leveraged to push storage technologies such as rechargeable batteries up to utility scale and economic viability.

Sodium sulfur, lithium ion and flow batteries are all the subject of intense research. Numerous startups have sprung up to bring the latest and greatest to the market. Additionally, there are initiatives around fuel cells, hydrogen power, superconducting magnetic energy storage and numerous other inventions that hope to unlock low-cost, large-scale storage.

But perhaps some of the answer lies in old-school turbomachinery approaches that rely on basic mechanical principles. Why reinvent the wheel when solutions are already available? Pumped hydro storage and compressed air energy storage are both proven in the field at the size needed to support the scale of modern solar projects and wind farms.

Taking a look at the capital costs for the various forms of energy, pumped hydro and compressed air came out well ahead of battery storage in the last Lazard study (December 2016) that directly compared these traditional approaches with battery storage in terms of $ per MWh.

Of course, battery storage prices have been declining steadily since that time. Lazard estimates that the rate of price decline ranges from 4% to 11% per year, depending on the battery technology. But there is bad news on that front. While prices have been dropping, the U.S. only experienced a 6% fall in battery prices in 2018 despite the market nearly doubling (350.5 MW of battery installations), according to energy consultancy Wood Mackenzie. 2019 figures are being finalized but are likely to easily surpass the previous year.

Perhaps it’s time to take another look at those golden oldies, pumped hydro and compressed air energy storage.

Pumped hydro

Pumped hydro storage (PHS) has been around since the tail end of the nineteenth century. Energy is typically stored by pumping water up a hill during low demand periods and storing it in a reservoir until it is needed during periods of high electricity demand or higher prices. Water is released to turn turbines that generate electricity in the same way power is generated in hydroelectric plants.

PHS can be constructed at a scale of hundreds of megawatts. The stored power can be released over a period of many hours. Despite all the headlines about battery and storage innovation, pumped hydro still accounts for about 90% of energy storage capacity in the U.S – 50 facilities adding up to 22 GW. Europe has about double that amount.

For example, Portugal has been adding PHS to handle the huge influx of renewables in recent decades. Two facilities take advantage of existing dams to store energy to address a growing problem – sudden shifts between abundant renewable resources and power demand.

The Frades I plant in the northwest of Portugal began in 2005 with an output of almost 200 MW. Its success led to the construction of Frades II utilizing two existing dams built for hydropower. Power is generated in a powerhouse built inside a cavern which holds two single-stage Francis pump-turbines, and two 420 MVA variable speed asynchronous motor-generators. Frades I and II now work in tandem to provide anywhere from 50 MW to 1,000 MW as required by the grid and the availability of renewables.

Voith Hydro and Siemens collaborated in the installation and commissioning of equipment. Voith supplied Frades 2 with both variable speed pump turbines (each with a rated output of 390 MW), two asynchronous motor generators (440 MVA each), the frequency converter and control systems as well as the hydraulic steel components. The company is currently introducing additional upgrades to extend the facility’s power range through integration of hydraulic short circuit technology for variable speed machines. This should help to increase dispatchability while implementing synthetic inertia and frequency containment reserve. Voith believes these upgrades can make Frades 2 a showcase for the potential of PHS to help Europe achieve its renewable energy and emissions targets.

PHS won’t work everywhere, however. Large reservoirs are required and not all regions possess them. But in places where hydroelectric facilities already exist, PHS is an option. Australia is reported to have more than 20,000 potential pumped hydro sites. A study by Australian National University (ANU) found that building only a dozen or so of those could help the nation transition to 100% renewables in less than two decades.

“No matter where you are in Australia, you will find a good pumped hydro site not very far away from where you, or your wind or your solar farm is located,” said ANU engineering professor Andrew Blakers.

Another interesting take on PHS comes from Michigan Technological University. Research in Michigan’s Upper Peninsula is considering abandoned metal mines as a possible home for pumped hydro. These mines tend to be flooded with groundwater, which could be pumped through turbines to generate power. A pilot project in the town of Negaunee, Michigan is ongoing. If this goes well, it opens the door to PHS across the American West which has hundreds of thousands of old mining sites.


Compressed air

Another alternative storage technology is Compressed Air Energy Storage (CAES). CAES uses low-cost electricity to inject air at high pressure into underground sites such as salt caverns, disused mines and oil fields and certain geological formations. When demand is high or renewables fall off the grid sharply, high-pressure air can be released and used to power natural gas-fired turbines, using much less gas than in a normal power plant.

Where CAES exists, it is a resounding success. However, lack of funding has inhibited its deployment. There are currently only two major examples in the field. Both utilize a single shaft design consisting of a compressor/expander, a clutch and a motor-generator.

The original CAES facility is in Huntorf, Germany. The 320 MW E.ON Kraftwerke plant was built in 1978. Its two salt caverns have a capacity of 310,000 m3. Its 60 MW compressor can fill the repositories in about eight hours, while the 290 MW turbine operates for two to three hours at an air mass flow rate of 417 kg/sec. Intercoolers on the compressor reduce energy requirements for compression.

The success of Huntorf gave rise to a similar plant being commissioned in the U.S. in 1991. The McIntosh plant in Alabama comprises three Dresser-Rand (now Siemens) compression trains and a huge salt cavern. Its compressor can produce about 54 MW and its expander is rated at 114 MW.

It takes about 40 hours to compress the chambers. Once done, the generator can operate at full capacity for 25 hours or so. The unit can perform emergency starts in nine minutes. The plant runs year round. It provides peaking power when needed and otherwise helps control the grid in fall and spring, or provides backup power. The facility responds based on market conditions. Fuel costs, electricity prices and grid conditions determine how it operates.

The German and U.S. CAES plants both use a single powertrain, with self-synchronizing clutches manufactured by SSS Clutch. The engagement or disengagement of these clutches controls whether the motor/generator is used to drive the compressor or generate grid power.

Since McIntosh, a series of additional U.S. CAES facilities have been proposed. The most recent is the 324 MW Bethel Energy Center in Anderson County, Texas. With a potential storage capacity of around 16,000 MWh, it will provide full power within 10 minutes from a cold start. Its purpose is to offer fast-response ancillary services to help balance renewable energy production, and storage of up to 48 hours to support the growing solar and wind fleet in Texas. It can provide the same volume of ancillary services as 2,000 MW of combined cycle gas turbines with only 112 MW of associated energy production. The project is fully permitted and construction-ready but awaits funding.

There are many suitable storage locations for CAES throughout much of the world. According to the EPRI, about 85% of the U.S. has geologic sites that could work for CAES. Northern Europe, too, has plenty of potential sites, some of which are already used for natural gas storage.


A modern update to traditional approaches to storage comes in the form of CryoBattery from Highview Power. Its cryogenic system uses liquid air as the storage medium to support power generation, provide stabilization services to transmission grids and distribution networks, and act as a source of backup power. These systems are clean – air in, and nothing but air out with standard plant configurations of 50 MW/500 MWh that can be scaled up.

When air comes in, it is compressed to form a liquid, refrigerated and stored in tanks (as opposed to requiring underground caverns). When needed, the liquid air is expanded into gas using a turboexpander and clutch arrangement to generate power. This technology is best suited for long-duration applications.

Highview Power has announced CryoBattery projects in Spain, the Middle East and South Africa. The company is also working on a number of projects in the UK (North Yorkshire) and in the U.S. (Kansas). For example, Highview is working with TSK, a Spanish engineering, procurement and construction (EPC) company, to co-develop gigawatt-hour scale cryogenic energy storage system.

“This partnership will help Highview Power accelerate momentum for our cryogenic energy storage systems in global markets,” said Javier Cavada, CEO of Highview Power. “It is ideal for applications like renewable energy shifting, enabling wind and solar for baseload generation, and hybridizing cryogenic storage plants with traditional thermal generation systems.”

Highview Power earlier built two cryogenic plants for the UK grid. One in Slough, Greater London has a capacity of 2.5 MWh. The other in Bury, Greater Manchester has a capacity of 15 MWh. Now the company is modularizing its technology for broader usage. It has partnered with Citec of Finland to streamline engineering and design to be able to deploy CryoBattery systems more efficiently and cost-effectively. The design includes a turboexpander attached to a clutch.

Conventional funding

A lot of money is being spent on battery storage R&D and incentives. This is certainly needed to push the technology forward. But perhaps some of those funds could be allocated to pumped hydro and CAES. The Australian government, for example, has approved close to $10 million for a startup named Hydrostor that has developed a new wrinkle on CAES. It aims to build a 5 MW demonstration plant in South Australia that takes surplus grid electricity and uses it to pump air into a cavern partially filled with water. Like the Upper Michigan Peninsula PHS pilot, it seeks to take advantage of a discontinued mine. Hydrostor claims to have 15 more sites in its development pipeline.

The large-scale deployment of CAES and PHS and other systems could offer the support wind and solar need to dominate the grid. Instead of dumping overcapacity at times when wind or solar generation exceed demand or transmission capacity, that power could be harnessed to store energy for use when demand is higher.

Alex Nicholson is a writer and energy consultant based in Southern California. He has a Masters Degree in Mechanical Engineering from Glasgow University.