Anthony Price, Innogy Technology Ventures Limited, Swindon, UK
The ‘Electricity Warehouse’ is not another dot.com service offering cheap electricity to internet customers, but it could change the way the electricity supply system operates.
Power systems have to be built so that at any particular moment the system’s generation exactly matches total demand (including losses on the network). If the two sides of the equation do not balance the results are predictable, but potentially catastrophic: the voltage changes, the frequency varies and if the system is not stabilized the network will crash.
Balancing generation and demand gives rise to additional fuel costs, as plants are held in reserve against an event that may or may not happen. There are also efficiency penalties if plants are used below their optimum output and the instantaneous cost of power varies during the day as different generating plants with different fuel types are used on the system.
Power system planners must install enough generation to meet peak demand (and ideally a little extra to accommodate contingencies). Extra transmission and distribution assets also need to be installed to ensure that the maximum load can be carried. Often these resources are duplicated to ensure security of supply in the event of a line failure. The result is an under-utilization of assets.
System planners have achieved the dream “just in time” process for matching demand and supply, but have done so out of technical necessity, rather than economic efficiency. Most other distribution networks include warehousing or storage, but in the power sector, storage plants are relatively rare. The most common type of storage is pumped hydro, which is capable of delivering hundreds of megawatts of power for several hours. This makes it ideal for use in centralized applications, providing reserve or peaking power.
Figure 1. Energy storage technology can provide the security of a reserve, avoiding the need for another standby plant with associated fuel and maintenance costs
On a smaller scale, the end users of power, such as commercial or industrial companies, can use storage devices, configured as interruptible power supplies, to ensure the quality and reliability of their power service. In the mid-range there are some emerging technologies that can offer both characteristics: the ability to respond to both short and long term variations in the balance between supply and demand.
An ‘Electricity Warehouse’ system being constructed by Innogy at the Little Barford power station in the UK is an example of one of these new plants (see PEi July/August 1999). The plant is capable of storing 120 MWh of energy and discharging it at a power of up to 15 MW. Half of the storage capacity is reserved to “black start” the power station. The remaining 60 MWh of energy storage capacity is available for other energy storage duties such as “time-shifting”, rapid response and voltage control. The plant uses the Regenesys energy storage technology (see inset box).
The way forward
The next stage in the commercialization of the Regenesys energy storage system is now underway. The value of storage, packaged as the Electricity Warehouse, is being recognized by an increasing number of utilities, power companies and end users of power. It has now been adopted by the Tennessee Valley Authority (TVA) of the USA. TVA operates 28 000 MW of generating plants, including 3000 MW of pumped hydro storage, as well as running the transmission network within its seven-state service territory.
The project is a collaborative venture between TVA and Innogy involving the design, supply and integration of fuel cell modules into a 120 MWh plant at a site in Mississippi, USA. TVA hopes to reinforce the power system in an area of weak distribution. Construction of the $25 million plant could start as early as spring 2001.
TVA Chairman Craven Crowell said: “The Regenesys system is cutting-edge technology that will help us meet the growing peak power demand in the future. This project demonstrates TVA’s leadership role in finding innovative ways to benefit the production and delivery of electricity in the US.”
The system exhibits many characteristics shared by most large power companies and so the project will be a good opportunity to demonstrate the application and benefits of energy storage. The options for improving reliability that the TVA considered included:
- Upgrading transmission and distribution systems
- Adding primary generation, such as peaking combustion turbines or diesel gensets
- Adding energy storage.
The last option turns the Electricity Warehouse into a reality. A storage plant can be charged, on a scheduled or unscheduled basis, and discharged to meet system requirements for short or long-term, real or reactive power. Even if the system is not required, storage can still provide the security of a reserve, avoiding the need for another standby plant. The facility to be constructed in the TVA area will not only provide additional power support, but will also provide information on the optimum way to operate the storage plant to maximize its efficiency for power producers and their customers.
Other sites considered for energy storage projects are being developed in North America and Europe. A Regenesys plant could be used at many places in a transmission and distribution network and a number of follow-on plants are expected to be built and operated in the coming years. There are a number of proposals for the use of energy storage as part of an energy trading activity: at large scale (25 MW/300 MWh) plants and at small scale (5-10 MW) associated with a cogeneration or industrial power plants.
Other applications for energy storage systems include the control of distributed resources and the integration of renewable energy sources, especially wind power. Storage and wind generation are complementary as the storage plant can be used to time-shift the energy output of the wind generator to increase its value, and to maintain frequency and voltage.
Just as a warehouse has to store an optimum amount of goods, an energy storage plant has to be optimized for its energy and capacity rating. One attractive feature of the Regenesys technology is the capability of long discharge periods (10, 20 or 30 hours) which makes it very suitable for integration in wind energy projects.
Energy storage cannot replace generation completely, but it can complement other forms of generation on a large or small power system, a stand-alone system or a non-grid connected application.
The addition of new peaking generating capacity could be deferred or avoided if the use of existing mid-merit generating capacity was increased and energy storage added to the system. Existing mid-merit capacity could be run at an increased load factor by storing the energy produced during off-peak periods. During peak periods, the energy could be discharged to meet peak demands.
The analysis of storage when used as a generation asset therefore needs to encompass the costs of mid-merit generation at high load factors, and allow for low load factor plants that can be withdrawn from service.
An energy storage device can be used for many network applications. Storage can achieve multiple benefits by combining applications resulting in increased returns on investments.
An energy storage plant is a multi functional tool on a power system. Power system applications of energy storage such as energy management, provision of ancillary services, and integration of distributed resources and renewable generation have significant technical and commercial benefits. Modern forms of energy storage, such as those based on a regenerative fuel cell provide the capability to incorporate energy storage with sufficient power and energy thereby improving the technical and commercial operation of a power network.
Electricity Warehouse: the technology
The Regenesys technology was developed by an international team led by British scientists and engineers. The system stores electricity when demand and costs are low, and then releases the energy quickly as demand for power grows.
The Regenesys technology operates like a battery that uses regenerative fuel cell technology to convert electrical energy into stored energy. A regenerative fuel cell plant stores or releases energy by means of a reversible electrochemical reaction between two electrolytes. This enables the system to be charged from its primary power source and then discharged to the grid.
The electrical energy is converted into chemical potential energy by ‘charging’ two liquid electrolyte solutions – sodium bromide and sodium polysulphide – and pumping them through the regenerative fuel cells in to individual circuits separated by an ion exchange membrane. The electrolytes are readily soluble and are non-hazardous in handling and storage.
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 cells and electricity is produced. The conversion can be repeated indefinitely and with high turnaround efficiency.