The assumed economic life of the battery system is 30 years, requiring battery component replacements at appropriate intervals. The structure and power conditioning system are expected to last 30 years . Battery replacement charges vary by the type of the battery and the number of years until replacement. One manufacturer claims that the type o f flooded lead-acid batteries they use should be replaced every three years . When VRLA batteries are used mor e widely for renewable applications in 2005, they initially are replaced at 5-year intervals, improving to 10-year intervals in 2010. Advanced batteries are assumed to require replacement once every 10 years when incorporated into the PV – battery system in 2020 [3,19]. This is an engineering estimate based on lifetime expectations for fundamental materials used in these battery systems and expectations for battery operation (charging and discharging).
The charging profile for the battery, which is pivotal in determining battery life, is controlled by the PCS for a grid – connected system. Continually undercharging a flooded lead-acid battery will cause it to sulfate, thereby greatl y reducing battery life. Overcharging a VRLA battery at moderately high rates and above will cause it to dry out, thereby reducing its life. Thus, the design and operation of the PCS is a major determinant of the system life cycle costs .
Battery energy storage systems operate at an AC-to-AC efficiency of about 75%, and, therefore, consume some energy. However, storage systems can accumulate energy during periods when efficient base load or renewable generation ar e
available, and discharge during peak load times, thereby reducing the use of less efficient peaking generators. AC-to – AC effic iency is the ratio of AC energy removed from a storage system to the AC energy used to charge the system. This efficiency measure includes all losses in the storage system from the battery, PCS, switchgear, etc. The AC-to-AC efficiency values are based on the existing performance of installed storage systems in the field. In the future, systems are expected to become more efficient through the use of improved storage devices and better power electronics. Th e storage device will become more efficient due to the use of improved technologies. The power electronics will b e enhanced through improved high-power switches that reduce losses . As shown in Table 1, AC-to-AC efficienc y increases from 76% in 1997 to 80% in 2010 and there after.
The annual energy delivery is calculated from the unit size and estimated operating time. Battery energy storag e systems are assumed to be available 90% of the time. Annual energy delivered is the projected amount from th e utilization of energy storage systems operated on average one hour per day for 100 days/year at 90% availability . Heavy-duty batteries of the type that should be used in solar plants can cycle daily up to 250 days per year .
The system energy footprint, measured in kWh/m2, is an important characteristic of storage systems, many of whic h will be installed in facilities with fixed and/or small areas available. The example 1997 baseline system is ver y compact: 1.5 x 1.5 m deep (2.3 m2) and 1.3 m high. The unit weighs 1,724 kg and can be located in service bay areas, warehouses or storerooms . The projected improvements in unit energy footprint are attributable to the expected increases in the energy density for VRLA and certain advanced battery technologies. The energy density of the VRLA, for example, is 15% greater than that of flooded lead-acid, hence the 15% increase in energy footprint.
The construction period is expected to be two months for PV array set-up; battery storage can be installed in a day o r less . The PV array is the only subsystem needed to be erected; all other components are contained in the modular, factory-assembled housing.