Energy Storage Technologies for Utility Scale Intermittent Renewable Energy Systems

In current conventional utility systems, fossil fuel or nuclear power plants must back up intermittent renewable electricity generators if the utility grid is to have acceptable reliability. For low levels of intermittent power, this can be achieved without difficulty or added expense. For high levels this reliance on other generators becomes explicit and increasingly expensive, and transmission capacity also becomes an issue. An attractive alternative to the conventional approach is one that relies on bulk or utility scale storage of intermittent electricity to provide system reliability. A comparison of the currently available storage technologies shows that the most cost effective and environmentally acceptable is a compressed air energy storage (CAES) system and especially its more advanced derivatives (CASH, CAES with Humidification and CAESSI, CAES with Steam Injection). For wind energy systems, both short term and seasonal storage are technically and economically feasible. Background It is increasingly obvious that anthropogenic global climate change must be taken seriously, and that carbon emissions must be reduced drastically if serious harm to the Earth’s biosphere is to be avoided. This certainly will involve dramatically reducing or eliminating the use of carbon based fuels to generate electricity and utilizing nuclear or solar energy systems instead. While nuclear power can technically be used to replace fossil fuels, it has important disadvantages, most significantly the irreducible finite possibility of a catastrophic accident. It would thus be desirable to limit its use to a minimum and satisfy most or all of our electricity needs using solar power. However, if solar generated electricity is to be a credible alternative it must have technical characteristics equal to those of fossil or nuclear power; that is, it must be easily utilized in modern industrial state, and its cost must be reasonable. Since renewable resources are generally diffuse, remote from major demand centers, and intermittent, the issues of transmission and storage must be addressed; fortunately, it is easy to demonstrate that an affordable technical solution to the challenge of electricity storage already exists. The need for utility scale storage is illustrated by the current status of wind energy, which now supplies about 15 percent of Denmark’s electricity. Wind turbines are by far the lowest cost and most successful source of renewable electrical energy available today. This is due both to the superb quality of the turbines developed over the past decade, as well as to far-sighted and effective public policy that mandates a justifiably high price for wind electricity. These same policies do have some negative effects, however, which up to now have not impeded the rapid increase of wind electricity onto the grid. Utilities, in most cases, are forced to absorb the costs of transmission line and substation reinforcement and of insuring overall system reliability. Given low wind turbine capacity factor (25-30 percent), transmission has already become an issue in some areas, while system reliability is increasingly a problem as wind penetration grows above 10 percent of average electricity demand, as it has in Denmark. In addition, a new system of balancing charges designed to insure that supply balances demand in deregulated markets threatens to penalize wind quite strongly [1]. One way to resolve these issues to the advantage of wind and other intermittent energy is to include storage on the system in a way that recognizes the wind/storage plant as a unified entity: that is, the output of the total system should be classified as renewable energy. This will resolve immediate transmission and reliability issues as well as allowing wind in the not too distant future to supply up to about 80 percent of total electricity demand [2]. Comparison Of Storage Technologies Pumped storage, batteries, superconducting magnet energy storage, flywheel energy storage, regenerative fuel cell storage and compressed air energy storage could be considered for bulk power storage; a cost comparison [3,4 ] of these is listed in Table 1. The critical parameters for these systems are