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.
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.
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
1. www.independent.co.uk/news/science/heating-vermiculitepotting-plants-heat-storage-summersun- winter-swansea-a7234406.html