National Power has developed a new regenerative fuel cell system and a 15 MW/120 MWh utility scale storage energy plant based on the technology is being built at Didcot. It is believed that this will be one of the largest energy storage plants of its kind in the world.

In the regenerative fuel cell electrical energy is converted into chemical potential energy by charging, and releasing the stored energy on discharge. National Power claims its Regenesys system is capable of being mass produced, with a target conversion efficiency of 75-80 per cent.

Regenerative fuel cells have inert electrodes that act 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 allows the complete separation of power, determined by the module electrode area, and energy, determined by the quantity of electrolyte stored in the tanks.

A regenerative fuel cell plant stores or releases electrical energy through a reversible electrochemical reaction between two salt solutions. The Regenesys system uses two electrolyte circuits containing sodium bromide and sodium polysulphide. These represent the starting conditions in the uncharged state, with sodium bromide on the positive side and sodium polysulphide on the negative side.

Economies of scale and manufacture can be achieved by linking cells together, with an electrode shared between two cells. The cathode of one cell becomes the anode of the next, known as a bipolar module.

The module only represents part of the storage system. A utility scale energy storage system uses many modules linked together to give the required power rating. Modules are linked electrically in series to form a string of the required DC voltage, and linked hydraulically in parallel. Additional strings are then added in parallel to give the required power rating of the plant. Electrolyte storage tanks of the required volume are added to establish the energy rating of the system. The storage capacity is only limited by the size of electrolyte tanks. A block diagram of the Regenesys system is shown in.


Following laboratory tests and the Aberthaw pilot scheme, the next stage is to build a full utility scale energy storage plant. The plant will be constructed at Didcot, and will be rated to provide 120 MWh of energy at nearly 15 MWe connected to the 33 kV distribution system.

Plant completion is expected in early 2001. During construction, the Regenesys modules, electrical equipment and other components will be delivered to site and installed in the module area. The electrolytes are delivered to site in a chemically stable and inert state and pumped into the storage tanks. In normal operation, they remain on site for the lifetime of the plant.

When commissioned, the plant will be able to start up in less than 10 minutes or, if held in stand-by mode with the modules filled with electrolytes, in less than 2 minutes. When running, the plant will be operated connected to the grid, and capable of turning from charging to discharging or any state in between in the order of 0.02 seconds. In stand-by or shutdown mode, there is no self-discharge of the electrolyte stored in the tanks.

The energy storage plant will be used to validate utility applications of energy storage such as:

  • Load following. Plant output can be varied to match load increases or decreases.

  • Voltage control mode. The store responds to fluctuations in AC system voltage, providing voltage regulation under steady state and transient operating conditions.

  • Frequency regulation. The store discharges up to its rated capacity in response to a fall in system frequency or frequency rate of change.

  • Power system stabilization. The store provides damping of power system oscillations in the range of frequencies 0-5 Hz by monitoring frequency fluctuations and controlling the store import/export.

  • Constant VAr. The system provides reactive power at a constant rate.

  • Constant AC power. The system charges or discharges at a constant AC power.

    A key component in any direct current battery or fuel cell system is the power conversion system (PCS). The PCS for the Didcot regenerative fuel cell project provides the interface between the 33 kV electrical supply and the variable operating voltage of the DC modules. The contract for the Didcot PCS supply was awarded to ABB Industrial Systems.

    The PCS consists of two functionally separate and autonomous converter systems, the chopper unit (DC/DC converter) providing the link with the variable voltage of the Regenysis modules and the DC/AC inverter unit, which is a 12 pulse three-phase three-level DC/AC converter. The two converter units are interconnected by a DC link with a fixed DC voltage level. A control unit also provides the means to adjust incoming and outgoing voltages and currents in real time to maintain the required energy exchange between the energy storage system and the grid. This results in a four quadrant converter system designed to transfer both reactive and real power simultaneously and independently from each other, according to the parameters set by the operator and within the capability of the PCS.

    The PCS allows the operator to select from a wide range of operating modes. The normal operating mode of the Didcot plant will be to follow a pre-defined schedule of current/voltage/time profiles during charge and discharge including the start up and shut down of the system. In parallel with this normal operating mode, the Regenesys system provides the user with alternative control modes.

    The future of energy storage

    Electricity is unique among commodity products, as it is very difficult to store. Because of this, electricity supply systems are built and operated to balance demand and production on a second-by-second basis.

    Because instantaneous demand for electricity must be balanced by production, system planners must build sufficient generation capacity to meet maximum demand plus a margin to ensure supply security. Because of demand fluctuations, some generating stations only need to operate for short periods. This is an inefficient use of expensive plant.

    Average demand for electricity in a typical system is about 60 per cent of maximum demand. To ensure that there is security of supply, a margin of capacity above maximum demand also has to be provided. As a consequence, the average utilization of the power stations is about 50 per cent, which compares with a potential utilization of some 90 per cent.

    Storage allows production to be decoupled from supply. One benefit of storage is that the production system can be optimised. If large scale energy storage is available, system planners would only need to plan to meet average demand rather than maximum demand.

    Storage can also significantly enhance the value of electricity produced by renewable generators, such as wind turbines, where instantaneous output cannot always be used to optimum advantage within the planned power plant scheduling of a network.

    Transmission companies would be able to increase the load factor of transmission lines. Storage can be used to stabilize power flows along transmission lines, and providing voltage and frequency support to the network.

    Distribution companies can use energy storage in order to replace or to defer investment in reinforcing their networks.