Battery Operation

The life of a b attery and its energy delivery capability are highly dependent on the manner in which it is operated. Many deep discharges (above 70-80%) reduce the life of lead-acid batteries. High rates of discharge reduce the energy delivery potential of lead-acid batteries. Batteries also have shelf-life limitations.

Poor charging practices are responsible for short battery life more than any other cause. A number of methods exis t for charging batteries used in stationary utility applications. Optimum life and energy output from batteries, but no t efficiency, are best achieved when depth of discharge (low, e. g., 40%) and time for recharge are predetermined and repetitive, a condition not always achievable in PV applications. Modified constant-potential charging is common for deep-cycling batteries and preferred for PV batteries designed for optimum life [3].

PV system manufacturers have incorporated battery storage into their off-grid installations for many years. Customers are beginning to request storage for grid-connected PV systems as well. The two systems have not been totall y integrated; redundant PCS and balance of plant exist since both the PV modules and battery systems generally com e with their own total package. The 1997 baseline system is derived from an existing 31 kW PV/21 kWh (40 minutes ) flooded lead-acid battery system that is currently being demonstrated at five different utility sites. The systems ar e located in Newark and Wilmington, DE; Northeast, MD; Green Bay, WI; and Aberdeen, NC [10]. Although none o f the sites have excellent solar insolation, there is good coincidence between peak solar generation and peak demand o f the host facility.

For this technology characterization, we assume a 30 kW system with one hour of storage available in the initial yea r and all outyears. The system is based on one module of a larger, commercially available (250 kW) power management battery system comprised of eight equally-sized modules. The 1997 system cost, benefits, and performance presented in Section 4.0 are based upon batteries and power electronics that are near-commercial today.

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