Energy_Storage_Technologies.pdf

Due to its commercial prevalence the rest of this

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Due to its commercial prevalence, the rest of this review will focus on NiCd chemistries. System Design Considerations Similar to lead acid batteries, NiCd batteries are typically available in predetermined sizes, and in larger energy storage systems they are typically connected in series/parallel combinations to get the desired power and energy capacity. These systems typically require and HVAC system to maintain the temperature, as well as sensors for monitoring for hydrogen gas [2]. Operation Overview of operations Self-discharge Since the electrolyte is somewhat conductive, a small amount of self-discharge occurs (and is typically mitigated through the application of a float charge). It should also be noted that the rate of self-discharge Self discharge significantly increases with temperature for this chemistry [2]. Lifetime Pocket-plate NiCd at 80% depth-of-discharge can last 800-1000 cycles, while at 10% depth-of-discharge it can last around 50,000 cycles. Sintered-plate NiCd at 80% depth-of-discharge may survive 3500 cycles. Nickel- metal hydride and nickel-hydrogen are on par with vented NiCd. Nickel-zinc last about 1000 cycles. For low numbers of cycles, flooded NiCd are rated for 10-15 years, NiFe may last 25 years [2]. Thermal Considerations These batteries must be kept at room temperature, because the internal resistance of the system is inversely proportional to temperature, so a higher temperature means lower internal resistance. So, as the temperature rises the internal resistance decreases, which leads to increased self-discharge, and decreased the overall lifetime of the device. Since operation at cooler temperatures result in higher charge capacity (in Amp- hours), it is considered optimal to store the battery cold when charged and warm it just prior to discharge [2]. Rule of thumb: calendar-life of NiCd batteries falls by 20% with every 10 C in operating temperature [2]. Float Charging Refer to Lead Acid Batteries in Section 3.6. Maintenance Issues Overview of maintenance issues Memory Effect Caused by repeated cycling at shallow depths-of-discharge, leading to a gradual reduction in both voltage and capacity at the end of the shallow cycle (remediated through a full discharge and recharge) [2]. Float Effect Causes Watt-hour capacity to decrease after too long on float-charge. This is remediated by 1-3 full charge/discharge cycles [2]. 21
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Energy Storage Technologies Nickel-electrode Batteries Irreversible Degradation Decomposition of organic materials in cell into carbonates, resulting in increased resistance in the electrolyte. Formation of dendrites on the negative electrode (which can penetrate the separator). Gas barrier failure which allows gases to recombine with the cell itself leading to heading, larger self-discharge rates, and eventually short-circuiting. Electrode poisoning (especially poisoning of the positive electrode with iron migrating from the negative electrode) [2].
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  • One '14
  • Energy storage

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