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Energy Storage Methodologies

Energy Storage Methodologies. Energy Storage. Energy storage mediums that store forms of energy that can be drawn upon at required time to perform some useful operation. Energy Storage.

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Energy Storage Methodologies

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  1. Energy Storage Methodologies

  2. EnergyStorage • Energy storage mediums that store forms of energy that can be drawn upon at required time to perform some useful operation.

  3. EnergyStorage • Energy storage as a natural process is as old as the universe itself - the energy present at the initial creation of the Universe has been stored in stars such as the Sun, • being used by humans Directly (e.g. through solar heating), or indirectly (e.g. by growing crops or conversion into electricity in solar cells). • Storing energy allows humans to balance the supply and demand of energy. • Energy storage systems in commercial use today can be broadly categorized as mechanical, electrical, chemical, biological, thermal and nuclear.

  4. EnergyStorage • Examples : • A wind-up clock stores potential energy (mechanical, in the spring tension), • a battery stores readily convertible chemical energy to keep a clock chip in a computer running (electrically) even when the computer is turned off, • a hydroelectric dam stores power in a reservoir as gravitational potential energy. • Ice storage tanks store ice (thermal energy)at night to meet peak demand for cooling . • Even food is a form of energy storage, chemical in this case.

  5. EnergyStorage (typical examples) Chemical : Hydrogen , Biofuels , Liquid nitrogen, Oxyhydrogen , Hydrogen peroxide. Biological : Starch, Glycogen. Electrochemical : Batteries, Flow batteries, Fuel cells,. Electrical: Capacitor, Supercapacitor, Superconducting magnetic energy storage (SMES) Mechanical : Compressed air energy storage (CAES), Flywheel energy storage, Hydraulic accumulator, Hydroelectric energy storage, Spring. Thermal : Ice Storage, Molten salt, Cryogenic liquid air or nitrogen, Seasonal thermal store, Solar pond , Hot bricks, Steam accumulator , Fireless locomotive

  6. Batteries • In electronics, a battery or voltaic cell is a combination of many electrochemical Galvanic cells of identical type to store chemical energy and to deliver higher voltage or higher current than with single cells. • The battery cells create a voltage difference between the terminals of each cell and hence to its combination in battery. When an external electrical circuit is connected to the battery, then the battery drives electrons through the circuit and electrical work is done.

  7. Batteries • Used as energy storage. • Lead-acid battery is commonly used as energy storage (UPS or PV system) • Other type of batteries – nickel-cadmium, nickel-metal hydride, lithium-ion, nickel-zinc and fuel cell. • Also provide other important energy services for PV system including ability to provide current surge that much higher than current from the array and automatic property of controlling the output voltage of array so that load receive voltages within their acceptability range. • Cheap, high efficiency but short lifetime.

  8. Performance considerations • Battery designs are available for the following standby applications: • Long duration (i.e., telecommunications or low discharge rate) • designed for applications in which the standby loads are relatively constant • required to supply these loads for a minimum of 3 h. (Long duration batteries are characterized by thicker plates) • b) General-purpose (i.e., switchgear and control) • similar to the long duration battery,but have additional design features to improve conductivity. • In UPS applications, this design is best suited for discharge times of 1 h to 3 h.

  9. Performance considerations • c) Short duration (i.e., UPS or high discharge rate) • are designed to supply large amounts of power for a relatively short period of time. • Thinner plates typically characterize short duration batteries. • best suited for applications requiring reserve times of 1 h or less.

  10. Battery storage capacity • Energy storage in battery is given in Amp-hours (Ah) unit at some nominal voltage and at some specified discharge rate. • A lead-acid battery has a nominal voltage of 2V per cell which means 6 cells for a 12-V battery. The amp-hour capacity at discharge rate that would drain the battery down to 1.75V over a specified period of time at 250C temperature. • Example: A fully charged 12V battery is specified to have 10 hours, 200Ah capacity could deliver 20A for 10h. At any point, the battery would have a voltage of 10.5V (6x1.75=10.5) and is considered to be fully discharged.

  11. Energy is volts x amps x hours but voltage varies throughout the discharge period. To avoid ambiguity, anything relate to battery storage capacity is specified in amp-hours rather than watt-hours. • A 200Ah battery is delivering 20A is said to be discharging at a C/10 rate; where C refer to capacity of Ah and 10h it take to deplete. • The amp-hour capacity depends on the rate at which current is withdrawn and also on temperature. • Long discharge times result in higher Ah capacity. • Battery capacity and output voltage decreases dramatically in colder condition. • A rule-of –thumb estimate is battery life is shortened by 50% for every 100C above the optimum 250C operating temperature.

  12. Lead-acid battery capacity depends on discharge rate and temperature. Ratio is based on a rated capacity at C/20 and 250C.

  13. Comparison of Battery Characteristics Normal car battery –SLI (starting, lighting & ignition)

  14. Impact of depth of discharge on no. of cycles on a deep-cycle lead-acid battery

  15. Example 1 • Suppose that batteries located at a remote telecommunications site drop to -200C. If they must provide 2 days of storage for a load 500Ah/day at 12V, how many amp-hours of storage should be specified for the battery bank? Max.Depth of Discharge

  16. Solution example 1 • From the figure above, to avoid freezing, the maximum depth of discharge at -200C is about 60%. For 2 days of storage, with discharge of no more than 60%, the batteries need to store:

  17. Since the rated capacity of batteries is likely to be specified at 250C at C/20 rate, we need to adjust the battery capacity to account for our different temperature and discharge period. From the rated figure above, the actual capacity of batteries at -200C discharged over 48h period is 80% of rated capacity. So, we need to specify batteries with rated capacity:

  18. Most PV battery system are based on 6V or 12V batteries, which may wired in series and parallel combinations to achieve the needed Ah capacity and voltage rating. • For batteries wired in series, the voltage add but since the same current flows through each battery, the amp-hour rating of the string is the same as it is for each battery. • For batteries wired in parallel, the voltage across each battery is the same, currents add, so the amp-hour capacity is additive.

  19. In below figure, there is no difference in energy stored in the two-battery series and parallel.

  20. Coulomb Efficiency • Battery capacity C is given in amp-hours and charging and discharging are expressed in C/T rates, also amps. • For example, a battery is charging with a constant current IC over a period of time ΔTC when applied voltage is VC. The input energy to the battery is: • Suppose that the battery is discharged at ID and VD over a period of time ΔTD, delivering energy:

  21. The energy efficiency of battery: • The current (A) x time (h) is Coulomb charge expressed as Ah, then the above equation becomes: • The ratio of discharge voltage to charge voltage – voltage efficiency of the battery and the ratio of Ahout to Ahin – Coulomb efficiency.

  22. Design and aging considerations • - Although it is possible to size a battery for a smaller connected load, the UPS battery is generally sized to accommodate the full-load rated capacity of the UPS. The installed battery capacity should be selected so that the battery is capable of supporting the full UPS load at the end of the battery life. • A lead-acid battery has reached the end of its life when the available capacity drops to 80% of rated capacity. Therefore, to ensure that the battery will support the fully loaded UPS for the entire life interval, •  the calculated battery size should be increased by a 25% margin.

  23. Blocking diodes • The simplest PV battery-system consists of a single module connected to battery and load. The system perform well if the user is careful not to let the battery discharge too deeply or be overcharged. • One disadvantage is that the system allows the battery to leak current back through PV module at night.

  24. The equivalent cct of a single PV cell shown below (a). Ignore impact of a very small series resistance and ideal source current because the cell is in the dark at night leaves a simple cct in (b).

  25. Current through diode is given by: • The nighttime current from the battery through each cell will be: • Where Vd across the diode will be equal to the battery voltage VB divided by number of cells, n in PV module. • With the simple nighttime equivalent cct, we can decide how much leakage will occur from battery through PVs.

  26. Example 2 Impact of a blocking diode to control nighttime battery leakage. • A PV module is made up of 36 cells, each having I0=1x10-10 A and Rp=8Ω. The PV provide the equivalent of 5A for 6 hours each day. The module is connected without a blocking diode to a battery with voltage 12.5V. a) How many Ah will be discharged from battery over a 12-h night? b) how much energy will be lost due to this discharge? c) If a blocking diode is added, how much energy will be dissipated through the diode during the daytime. Assume the conducting diode has a voltage drop of 0.6V.

  27. Solution • The voltage across each PV cell is 12.5V / 36 cells = 0.347V. • The discharged current from battery while PV is in the dark will be: • a) Over a 12-h nighttime period, the loss in Ah from the battery will be: Nighttime loss = .043 A x 12 h = 0.516 Ah • b) At nominal 12.5V, the energy loss at night will be: Nighttime loss = 0.516 x 12.5 = 6.45 Wh

  28. c) During the day, PVs will deliver: PV output = 6 h x 5 A = 30 Ah The nighttime loss without blocking diode is 0.516 Ah / 30 Ah = 0.0172; that is 1.72% of daytime gains. With blocking diode drop of 0.6V, the daytime loss caused by the diode is: Blocking diode loss = 30Ah x 0.6 = 18Wh The blocking diode loses more energy during the day while it is conducting (18Wh) than it saves overnight (6.45Wh) or without blocking diode, only about Ah=1.72% of daytime solar gains are lost overnight.

  29. Sizing PV array • In figure below, PV I-V curve has been drawn along with a vertical I-V line for a battery. • During battery charging, the operating point always above the knee of PV curve, which means that charging current will exceed the rated current of PVs. • The operating point for battery charging is usually some distance away from MPP. This means that a fraction of power that PV could provide based on rated power PR of the module is not being delivered to the batteries. • The product of IR x peak hours of insolation provides a good estimate for Ah delivered to the batteries.

  30. Ah delivered from the batteries to the load is: Ah to load = IR x peak sun hours x Coulomb efficiency x derating factor

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