The Hywind floating wind turbine farm in the North Sea incorporates energy storage from lithium-ion batteries. Depicted here, the first nacelle and rotor (blades on hub) being lifted on to the towers at assembly site for Hywind Scotland at Stord. Photo: Jan Arne Wold / Woldcam / Statoil.

Lift-off: energy storage brings renewable energy to the mainstream

In the past, a severe shortcoming of renewables has been that they are intermittent: the power stays on only as long as the wind blows or the sun shines; if the wind blows too hard, the turbines must stop turning in order not to overload the grid. Newer and more efficient storage techniques are changing the equation. It is now possible to store excess power and tap into it during periods of power shortage. This article outlines some of the many technologies employed.
 
^ The Hywind floating wind turbine farm in the North Sea incorporates  energy storage from lithium-ion batteries. Depicted here, the first nacelle and rotor (blades on hub) being lifted on to the towers at assembly site for Hywind Scotland at Stord. Photo: Jan Arne Wold / Woldcam / Statoil.

Article by James Chater
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Great leap forward

Energy storage looks set to become one of the defining technologies of the early 21st century. It will revolutionize power generation by integrating renewable energy into grids, evening out the peaks and troughs of energy production and making power generation more efficient overall.

The second half of 2016 proved a turning point as utilities, manufacturers and equipment providers started to pool their efforts. Whereas total capacity additions of non-pumped hydro utility-scale energy storage amounted to slightly over 500MW in 2016, the second half of the year saw announcements of 1GW of new capacity.

Limitations of pumped storage

The concept of energy storage is familiar to hydroelectric power. During periods when supply exceeds demand, water is pumped to relatively high ground; when demand exceeds supply, the water descends through a turbine to produce electrical power. This method, called pumped-storage hydroelectric power, still accounts for 96% of all energy storage.

What about storage of other renewables, such as solar and wind? It is possible to combine wind or solar power with pumped storage hydroelectric power, as the Naturstromspeicher Gaildorf project in Germany demonstrates. But pumped storage works best in mountains, so the role it can play in expanding storage capacity in the future is limited. The extra capacity must come from other technologies. In this article we take a look at a few of the most promising storage methods: thermal energy storage, batteries, HTS magnets, hydrogen, compressed air storage and Liquid Air Energy Storage.

Thermal energy storage

This is the process used to store heat from the sun’s rays in a CSP plant (see my survey on renewables in the August 2017 edition of Stainless Steel World). Molten salt is stored at a high temperature and, when required, coverts water into steam which drives a turbine. Stainless steel type 347 is the preferred material and is used in Heatric diffusion bonded heat exchangers for this application. A similar process exists using graphite instead of molten salt.

Batteries

The most commonly found energy storage battery, and one of the most efficient, is the lithium-ion battery. It is standard in many products, such as mobile phones, laptops and home energy storage batteries, as well as in countless renewable energy projects. A recent example is Statoil’s Hywind offshore wind farm, the world’s first floating wind farm, which will use Batwind technology developed by Scottish Enterprises.

Lithium-ion batteries will play an important role for many years to come, thanks to massive investments by Tesla, the latest being the just announced factory in South Africa. But lithium is expensive and in short supply. Various alternatives are being developed. The Redox-flow battery is an electrochemical device that uses a liquid storage medium and can vary enormously in size. It is suitable for both solar and wind power.

Zinc-ion batteries for grid energy storage have been developed at the University of Waterloo, Ontario, Canada. They feature non-flammable, non-toxic materials and a pH-neutral, water-based salt, and are said to cost half the price of current lithium-ion batteries. Also promising are organic flow batteries. One such device, developed at Harvard, uses quinones from cheap and abundant sources such as rhubarb or oil waste. This safe, clean way of storing power promises to bring storage costs to below USD 100 per kilowatt hour.

Highview’s LAES pre-commercial demonstration plant in Greater Manchester, UK.
Highview’s LAES pre-commercial demonstration plant in Greater Manchester, UK.

Stainless use in batteries

Since batteries have to be as light as possible, ultra-thin metal substrates are being developed for battery and ultracapacitor components. Nickel, aluminium and copper are currently used, but titanium and stainless steel are emerging as lighter alternatives.

HTS magnets

High-temperature superconductor magnet technology is a relatively new way of storing energy. The new technology, developed at Brookhaven National Laboratory, could be used in renewable energy storage and remote energy distribution. The coil technology employs stainless steel.

Hydrogen

Hydrogen is the most abundant substance, and also one of the simplest and cleanest. It is bound to play an important role in the transition to greener forms of energy. Hydrogen can be produced from any primary energy source, including renewables. The basic chemical process consists of splitting water into its two elements, oxygen and hydrogen. In order to produce electricity, H is made to react with O; the only two byproducts are heat and water. This reaction occurs within fuel cells. The hot gases and humid conditions of fuel cell stacks favour the use of ceramics and stainless steel. A recent development is the arrival of stainless steel micro fuel cells made by laser additive manufacturing.

The LAES process in three steps. Image: Highview Power Storage.
The LAES process in three steps. Image: Highview Power Storage.

Compressed/liquefied air

Another storage technology that uses stainless steel is liquid air energy storage (LAES), which is a cryogenic process. It can be compared to compressed air storage (but without the geographical constraints), which uses surplus power to compress air and then generates electricity later by expanding it through a turbine. The UK company Highview Power Storage has gone a stage further. It uses cheap, off-peak energy to cool air to -196°C so that it becomes liquid and which is 1/700th of its volume and is stored at low pressure in stainless steel tanks. When the energy is required, a tap is turned, the liquid air is heated, expands and drives a turbine.

The Highview technology uses Heatric printed circuit heater exchangers (made of type 304L stainless steel) to capture cold air at the moment when the liquid air warms and evaporates, sending it to a cold store for re-use in liquefaction. Other suppliers include GE (turbine and generator), Avintrans’s Stainless Metalcraft business (thermal storage tanks) and BOC (cryogenic storage tanks). The thermal storage tanks are made of an especially ductile carbon steel, EN 10028-1 P 265 GH (1.0425).

The cryogenic tanks consist of an inner layer made of type 304 stainless steel surrounded by a carbon steel layer (1). The same grade was also used for all the stainless pipes and for the high-grade storage tank. Highview built a pilot plant in Slough which ran on the UK grid for four years and is currently commissioning a 5MW demo plant with project partners Viridor in Greater Manchester. They are not alone in this field. Florida-based Keuka Energy has launched a vessel that combines floating wind turbines with LAES; compressed air projects are being run by Bright Energy and Hydrostor.

Did you know?

– Vermiculite, a type of salt used for potting plants, can be used to store energy. If you blow warm air over it, it will dry out. If you then expose it to cold, damp air, it absorbs water and releases heat. This is known as interseasonal heating (1).

– Researchers at the University of Washington at Seattle are developing a mobile telephone without batteries.

References

Printed circuit heater exchanger made of type 304L stainless steel from Heatric, used in the LAES process. Photo courtesy of Highview Power Storage and Heatric.
Printed circuit heater exchanger made of type 304L stainless steel from Heatric, used in the LAES process. Photo courtesy of Highview Power Storage and Heatric.
 

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