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Mena 1:00 Team R15 the grid while the remaining 25%-35% is converted to other forms of energy such as heat. Meanwhile, lead acid batteries only have an efficiency of about 45%, which indicates that about 55% of the energy stored in the battery is rendered unusable by conversion to other forms of energy [5]. Another benefit vanadium redox flow batteries have over lead acid batteries is self-discharge. Self-discharge is a measure of the percent of energy that is dissipated in a battery while not in use. Self-discharge can occur for a number of reasons, but mostly occurs when two or more of the species in a container react, converting the stored potential energy into chemical and heat energy [6]. Vanadium redox flow batteries manage to have a self- discharge of nearly 0%. They are able to accomplish this since the design of the battery keeps the active species in separate containers, which minimizes the ability of the species to chemically react when the battery is not in use. Meanwhile lead acid batteries have a self-discharge of 5%, meaning that, on top of the 55% of energy lost in efficiency, up to another 5% is lost while the battery sits idle [7]. Vanadium redox flow batteries also tend to outperform lead acid batteries in terms of energy density. Energy density is the amount of energy, measured in watt-hours, which is stored in one liter of the solution. Vanadium redox flow batteries have been measured to have an energy density ranging from 15-25 watt-hours per liter, but theoretically could have a value ranging from 30-47 watt-hours per liter. Lead acid batteries, however, have only been measured to have an energy density ranging from 12-18 watt-hours per liter. However, if more developments are made, lead acid batteries could theoretically have an energy density of up to 40 watt-hours per liter [5]. In addition to being more energy dense, vanadium redox flow batteries have a greater depth of discharge than lead acid batteries. The depth of discharge, which is the inverse of the state of charge, is a percent measure of how much charge is able to be discharged in one cycle of the battery [6]. The depth of discharge of a vanadium redox flow battery is around 75%, meaning it can discharge up to 75% of its current energy storage in one cycle. Meanwhile, a lead acid battery only has a depth of discharge ranging from about 25%-30%, meaning it can only discharge that amount of energy in one cycle [5]. One of the most notable benefits of vanadium redox flow batteries over lead acid batteries is that vanadium batteries are able to support many more life cycles, giving them a very long product life span. The number of life cycles it can run through is the measure of how many times the battery can be charged and discharged (one cycle of charge and discharge is equal to one life cycle) before a certain component of the battery decays to the point where it can only store 80% of the energy it could initially [8]. Vanadium redox flow batteries have been determined to produce over 10,000 life cycles, which generally corresponds to lasting between 15-20 years before 4 University of Pittsburgh, Swanson School of Engineering First-Year Conference Paper 29.03.2019 replacement is needed. Meanwhile, lead acid batteries can only produce about 1500 life cycles and have rarely been seen to last more than 5 years [5]. Furthermore, vanadium redox flow batteries avoid many of the negative environmental consequences associated with lead acid batteries. The use of vanadium in vanadium redox flow batteries is much safer for the environment than other chemicals used in batteries, such as lead and cadmium, as it is significantly less toxic. Conversely, the use of lead in lead acid batteries is significantly more harmful to the environment than vanadium, as it is very toxic. However, the use of vanadium versus lead in the battery is not the only factor which contributes to vanadium redox batteries having a smaller environmental impact than lead acid batteries. The design of vanadium redox flow batteries, when used in stationary applications, produces only 7%-25% of the environmentally harmful emissions (CO2, SO2, CO, CH4, NOx) that lead acid batteries emit [5]. Cost comparisons between the two batteries suggest different results depending on which costs are being considered. When considering the upfront cost of the two types of batteries, the battery that is more expensive can depend on the specifications of the particular model of each battery being compared. What is consistent across models of each battery is that the maintenance costs of vanadium redox flow batteries are significantly less than the maintenance costs of lead acid batteries. The vanadium redox flow batteries have a maintenance cost of about $0.008 per kilowatt-hour, while lead acid batteries have a maintenance cost of about $0.02 per kilowatt-hour [5]. Shortcomings of Vanadium Redox Flow Batteries When Compared to Lead Acid Batteries Although vanadium redox batteries are fairly efficient and affordable form of grid energy storage, they do have a few shortcomings. For example, vanadium redox flow batteries are much larger than lead acid batteries. A 40-64V, 14kW vanadium redox flow battery has dimensions of about 0.75 meters long by 1.5 meters wide by 2.0 meters tall for a single cell. Meanwhile, a similar lead acid battery has dimensions of 0.314 meters long by 0.183 meters wide by 0.388 meters tall for a single cell [9][10]. Lead acid batteries also outperform vanadium redox flow batteries in terms of power density. Power density is similar to energy density but instead of measuring the energy (measured in watt-hours) it is the measure of power (measured in watts) per kilogram. Vanadium redox flow batteries have been measured to possess a power density of about 166 watts per kilogram. This is attributed to the limited solubility of the vanadium species, as well as its low specific energy densities. This low power density suggests that vanadium redox flow batteries are not desirable for systems that require high power per unit of volume, such asPDF Image | VANADIUM REDOX FLOW BATTERIES FOR GREEN ENERGY
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