Energy storage and wind energy conversion systems

The demands imposed by the variability of wind power input have pushed the technical performance and cost requirements for energy storage to the forefront. In principle, wind turbine generators can be integrated with almost any kind of energy storage technology. However, the most appropriate energy storage system depends on the system size and the type of energy being delivered. Because most commercial wind turbines are designed to deliver electrical power, the vast majority of systems that do incorporate energy storage use batteries. Large scale systems using batteries have not been shown to be viable. Up to now, most efforts and successes in reliably and economically integrating storage with wind turbine generators have been concerned with relatively small power plants, less than about 1 kW. Very small wind/storage systems, where the wind turbine is dedicated to charging conventional lead–acid batteries, currently dominate the market for wind power/energy storage hybrid power systems. These systems are well developed and proven. So–called 'village scale' hybrid power generation systems using energy storage (about 5 kW to 100 kW capacity) are now the subject of considerable product development and commercialization. The key technical problem for off–grid or stand–alone wind power systems of this size that must deliver well– regulated (i.e. high quality) AC power may well be to implement the most reliable and cost effective short term and highly responsive energy storage systems (on the scale of 1 second to 10 minutes). Such storage schemes must respond effectively to the wind power fluctuations and the load demand fluctuations to maintain network stability. Pumped hydro and underground compressed air storage will probably find some applications in larger scale wind power plants where they can work with the wind power plant to deliver baseload power to the grid, however the number of applications for these systems is limited because of the specific geological and geographical requirements of the storage devices. Furthermore such plants are enormously capital intensive and are usually competing with conventional power plants on well–integrated regional and national grids in developed countries. Therefore projects of this type are unlikely to move forward until the economics are compelling to investors and lenders. Technical feasibility is not an issue. Configuring the best hybrid wind power system for a given application is difficult. The existing load demand characteristics and expected long–term load and the wind resource must be well characterized and understood before a system can be designed. The need for energy storage will be determined through the process of system configuration analysis after these data have been acquired. Because of the high costs associated with customized designs, wind storage hybrid power systems will likely only see broader application in village scale and larger systems when 'packaged' standardized designs are available and proven. The very simple micro–scale wind battery systems, of which hundreds of thousands are deployed, are an example of the effect of a standardized design 'package' on market penetration. Flywheels, high pressure hydraulic storage and ultra–capacitors could play a significant role in commercial systems as short term storage for power and load levelling. High penetration no storage systems may be a viable solution for larger scale off–grid applications where the cost of conventional large–scale storage (such as batteries) is prohibitive. Wind/storage hybrid systems and other renewable hybrid power plants can be very complex. Experience to date shows that overcomplicated systems will fail to perform satisfactorily. Simple, robust designs, combined with good training and a good inventory of spare parts and tools, are a necessity for successful stand–alone power systems.