This appendix assumes that current R&D activities will lead to significant improvements in the cost and performanc e of battery storage systems. As these improvements take place, battery storage systems will compete with conventional sources of peak electric power generation, such as gas turbines, diesel generators, or uninterruptible power supply units. Flooded lead-acid and VRLA batteries are commercially available today, although not in designs wholly suited to utility applications. Zinc/bromine and lithium batteries are two advanced batteries under development. Each of thes e technologies has particular strengths and weaknesses.
Lead-Acid Batteries: Basically, flooded lead-acid battery technology for renewable energy storage systems is th e large-scale application of a technology similar to that found in automobile batteries. Flooded lead-acid batteries ar e manufactured in large numbers for many uses and their operating characteristics and technology are well understoo d by manufacturers. However, they have several key limitations: (a) they require relatively frequent maintenance t o replace water lost in operation, (b) they are relatively expensive compared to conventional options with limite d reduction in cost expected, and (c) because of their use of lead, they are heavy, reducing their portability and increasing construction costs. The strengths of flooded lead-acid batteries center around their relatively long life span, durability, and the commercial availability of the technology. This allows flooded lead-acid battery customers to better justify their acquisitions and to amortize the cost of their systems over a longer period. Flooded lead-acid batteries are the most common batteries found in PV applications.
VRLAs: VRLAs use the same basic electrochemical technology as flooded lead-acid batteries, but these batteries are closed with a pressure regulating valve, so that they are essentially sealed. In addition, the acid electrolyte i s imm obilized. This eliminates the need to add water to the cells to keep the electrolyte functioning properly, or to mi x the electrolyte to prevent stratification. The oxygen recombination and the valves of VRLAs prevent the venting o f hydrogen and oxygen gases and the ingress of air into the cells. The battery subsystem may need to be replaced more frequently than with the flooded lead-acid battery, increasing the levelized cost of the system. The major advantage s of VRLAs over flooded lead-acid cells are: a) the dramatic reduction in the maintenance that is necessary to keep th e battery in operation, and b) the battery cells can be packaged more tightly because of the sealed construction an d immobilized electrolyte, reducing the footprint and weight of the battery . The disadvantages of VRLAs are that they are less robust than flooded lead-acid batteries, and they are more costly and shorter-lived. VRLAs are perceive d as being maintenance-free and safe and have become popular for standby power supplies in telecommunication s applications, and for uninterruptible power supplies in situations where special rooms cannot be set aside for th e batteries .
Advanced Batteries: Among the advanced batteries which may support renewable energy applications is th e zinc/bromine system. It uses a flowing aqueous zinc bromide electrolyte, with metallic zinc being deposited on th e negative electrode, while the bromine produced at the positive is stored in external tanks. The advantages o f zinc/bromine battery technology are low cost, modularity, transportability, low weight, and flexible operation. Because of the chemical nature of the reactants and room-temperature operating conditions, the casing and components can b e constructed from low-cost and light-weight molded plastic and carbon materials. The major disadvantages o f zinc/bromine batteries center around the maintenance requirements, including upkeep of pumps needed to circulat e the electrolyte, and the somewhat lower electrical efficiency. Also, the zinc deposited during the charging process must be completely removed periodically .
Other advanced batteries include the lithium-ion and lithium-polymer batteries which operate at or near ambien t temperatures and may become appropriate for renewable energy applications. Rechargeable lithium batteries hav e already been introduced into the market for consumer electronics and other portable equipment in small button an d prismatic cylindrical sizes . The advantages of lithium batteries include their high specific energy (four times tha t of lead-acid batteries) and charge retention. However, scaling up to the sizes, power levels and cycle life required fo r large applications remains an exacting challenge.
Technology development currently underway (with assistance from the DOE-SNL-ESS program among others) i s expected to significantly improve the performance and reduce the operation and maintenance (O&M) costs of energ y storage systems. Engineering development is proceeding on VRLA battery systems, which are nearly commercial, and advanced battery systems, which may be near-commercial within 10 years. Government and private industry ar e currently developing a variety of advanced batteries for electricity, transportation, and defense applications: lithiu m ion, lithium polymer, nickel metal hydride, sodium metal chloride, sodium sulfur, and zinc bromine. The large cos t of development of these new technologies is being shared by many organizations world-wide.