National Power (U.K.), has announced a new electricity storage technology – called Regenesys – in which a flowing electrolyte is charged and then and stored in tanks for later use. It has a high speed of response, supplies real and reactive power and is therefore suited to many different applications on a power system.

The Regenesysª system is based on regenerative fuel cell technology, (sometimes known as redox flow cell technology). Two electrolytes flow through the fuel cell on either side of an ion exchange membrane. By applying a voltage across the electrolytes they change state and become “charged”. The “charged” electrolytes pass out of the fuel cell to be stored in tanks. Just like a rechargeable battery, the process can be easily reversed. The “charged” electrolytes flow back through the fuel cell and electricity is produced.

The two electrolytes are concentrated solutions of sodium bromide and sodium polysulphide. The technology is environmentally benign, modular, comparatively easy to site, and separates the power rating from the energy storage capacity. These features make it suitable for energy storage applications in the 5 – 500 MW range which require storage times from fractions of a second to 12 hours or more.

Following successful trials of a Regenesys pilot plant at a power station in South Wales, the company will build its first full scale commercial plant at Didcot in Oxfordshire. Detailed designs are now complete for up to a 15 MW and 120 MWh utility scale energy storage plant. The plant would be housed in a low-rise building, occupying a compact site conservatively estimated at less than 0.5 hectare (1.2 acres). This generic design could be used for a number of applications within the power industry.

The total installed capital cost will be approximately $150/kWh. With continued technical improvements, National Power has set an eventual target price of US$80/kWh.

A storage plant with these cost and performance characteristics will provide significant technical and financial benefits in the operation of a network, from more efficient use of plant (generation, transmission and distribution), and from improved system performance. Storage can also significantly enhance the value of electricity produced by renewable generators, such as wind turbines.

The only available existing large-scale energy storage techniques are pumped hydro or compressed air energy storage, which have severe geographical limitations. Regensys would provide a real alternative. Other energy storage techniques such as batteries, flywheels, superconducting magnetic storage and supercapacitors have different capacity characteristics, and are not well suited to large scale applications.

National Power has formed a new business unit within its Commercial Division to develop the Regenesys technology in the UK and overseas.

Contact: Barry Davidson barry.davidson@natpower.com tel 011-44-1235-444-991

http://www.national-power.com/regenesys/brochure_FSET.htm

(I also have a PDF file of their brochure)

==== Some Additional Technical Details ===============

Regenerative fuel cells are a separate class of electrochemical device, which have inert electrodes acting only as an electron transfer surface. The electrodes do not take part in the electrochemical process and so do not limit the energy storage capacity of the regenerative fuel cell. This approach allows the complete separation of power, determined by the module’s electrode area, and energy, determined by the storage tank volume.

There are many electrochemical couples that have been assessed for use in flow battery systems. The Regenesys system uses electrolytes of concentrated solutions of sodium bromide and sodium polysulphide. These salts are readily soluble and present no adverse hazards in handling or storage. They are abundant and available at the necessary degree of purity at moderate cost. The use of other bromide and sulphide salts was investigated during the development phase, but the increased electrochemical efficiency would not necessarily repay the additional costs of the alternatives.

The simplified overall chemical reaction for the cell is given by:

3 NaBr + Na2S4 2 Na2S2 + NaBr3

The conversion of electrical to stored chemical energy and back again can be repeated indefinitely with high turnaround efficiency. There is no memory effect associated with the specific electrochemistry of the Regenesys system, and a full charge/discharge cycle can be completed without limitation of a theoretical maximum depth of discharge.

When commissioned the plant will have the ability to start up in less than 10 minutes or, if held in stand-by mode with the modules filled with electrolytes, in seconds. The plant will have a high rate of dynamic response. When running, the plant will be operated fully connected to the grid, capable of turning from a state of fully charging to fully discharging or any state in between in the order of 0.02 seconds. This performance makes the plant suitable for a number of ancillary service applications such as voltage control and frequency response. In stand-by or shutdown mode there is no self-discharge of the electrolyte stored in the tanks.

The Power Conversion System (PCS) provides the interface between the AC network electrical supply and the variable operating voltage of the DC modules. The four quadrant converter system is designed to transfer both reactive and real power simultaneously and independently from each other.

The PCS allows the operator to select from a wide range of operating modes.
– Pre-defined schedule
– Load following
– Voltage control mode
– Frequency regulation
– Power System Stabilisation
– Constant VAr
– Constant AC power
– Self-commutated to operate as a UPS, or to provide Black Start

And, practical peak shaving and dispatch optimization on networks, which has been limited by the availability of suitable technology.