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Energy_Storage_Technologies.pdf

Some devices every square centimeter of electrode

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some devices, every square centimeter of electrode consists of one to two thousand square meters of surface area [2] - this significantly increase the capacitance, and thus energy storage capacity of the device over conventional capacitors. Uncharged Cations (positively charged ions) + _ Anions (negatively charged ions) + + + + + + + _ + + + + + + + + + + + + + + + + _ + + + + + + Negative Charge (electrons) Positive Charge Charging Charged Electrolyte _ + + _ _ _ _ _ _ _ _ _ _ _ _ _ _ + + + + + + _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ + + + + + + + + _ _ _ _ _ _ _ _ _ + + + + _ _ _ _ _ + _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ + + + + + + + _ DC Power Supply _ _ _ _ _ _ _ + + + + + + Figure 3.3: Stages of charge in an electrochemical capacitor 16
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Energy Storage Technologies Electrochemical Capacitors (aka Supercapacitors) Variations There are symmetric and asymmetric designs, referring to the similarity of the two electrodes, and aqueous and organic electrodes [2]. These lead to four potential configurations, the where the differences between the performance of each is nuanced, so this is just mentioned for the interested reader. System Design Considerations Due lower single-cell voltages of about 6 Volts, hundreds of these cells have to be connected in series to achieve higher voltages. This can be a serious problem for larger system designs, since the typical failure mode for a cell is an open circuit. If a single device fails then the entire system may fail. This presents a reliability risk to be factored into the design of the system. Another consideration is due to potential damage due to placing a higher-than-rated voltage across a cell, since, unlike batteries, electrochemical capacitors cannot deal with gassing or the drying-up of electrolyte from electrolysis. To keep the voltages within safe operating limitations, resistors or Zener diodes may be connected in parallel and/or the voltage and state-of-charge of each device can be monitored and charged or discharged individually [2]. Operation & Maintenance One of the biggest advantages of electrochemical capacitors over batteries is the ability to charge and discharge more quickly (since there is no waiting for a chemical reaction to occur). And can practically be charged at any rate as long as the system stays within its designed temperature range, which is -55 C to 85 C. Environmental Impact There are little to no negative environmental impacts of these devices. Other Resources Overviews of electrochemical capacitors include [2,8,13], and in more detail [15]. Summary of Device Parameters The following table summarizes the available technoeconomic parameters for electro- chemical capacitors (EDLC parameters presented here) from a number of studies from 2000-2010. All monetary values have been adjusted to 2010 dollars. Electrochemical Capacitors Source: Schoenung EPRI Gonzalez Schoenung Chen 2003 [5] 2003 [2] 2004 [3] 2008 [6] 2009 [7] Techno. Params. Roundtrip Efficiency [%] 95 - 90 - 90-98 Self-discharge [%Energy per day] - - - - 20-40 Cycle Lifetime [cycles] - - 10k - 100k+ Expected Lifetime [Years] n/a - - - 20+ Specific Energy [Wh/kg] - - - - 2.5-15 Specific Power [W/kg] - - - - 500-5k Energy Density [Wh/L] - - - - - Power Density [W/L] - - - - 100k+ Costs Power Cost [$/kW] 360 - 350 350 100-300 Energy Cost [$/kWh] 36k - 94k 500 300-2k PCS Cost [$/kW] - 180 270-580 - - BOP Cost [$/kW] - 120 12k 50 - O&M Fixed Cost [$/kW-y] 6.0 14-16 6.4 - - Electrostatic or Electrolytic Capacitors Source: Chen 2009 [7] Techno. Params.
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