Lesson 17: Energy Storage PDF

Lesson 17: Energy Storage Overview There are a variety of technologies by which excess electric energy can be stored for later use. Some of these technologies charge and discharge electricity over periods of seconds or shorter, while others do so over many hours. Furthermore, some of these technolo...

Lesson 17: Energy Storage Overview There are a variety of technologies by which excess electric energy can be stored for later use. Some of these technologies charge and discharge electricity over periods of seconds or shorter, while others do so over many hours. Furthermore, some of these technologies draw energy from non-electric sources and return it as electricity, such as pumped hydro plants, while others store energy from electricity and re-release it as another form of energy, such as ice storage for cooling. The most common means of storing energy for use later is via batteries. Thus, after reviewing the main types of energy storage and their niche uses, the rest of this lesson focuses on how batteries work and perform. The lesson ends with a brief overview of hydrogen fuel cells, which themselves may end up having a significant niche use if the technology becomes more economic. Lecture Outline 1. The major forms of energy storage a. Electrochemical, e.g. i. Batteries ii. Hydrogen fuel cells b. Electromagnetic, e.g. i. Capacitors c. Thermodynamic, e.g. i. Compressed air ii. Ice iii. Molten salt d. Mechanical, both potential and kinetic, e.g. i. Pumped hydroelectric dam ii. Flywheel 2. Energy storage further classified by a. Max energy output b. Max power output 3. Battery basics a. There are three main components in batteries i. Negative anode ii. Positive cathode iii. Electrolyte, which separates two electrodes b. When a conductor is connected to the two electrodes i. Electrons move from the anode to the cathode through the conductor ii. Ions move between the electrodes through the electrolyte c. When a battery is charged i. Electrons flow through a conductor from the cathode to the anode ii. Ions move back to their respective electrodes through the electrolyte d. Battery efficiency is 80-85% due to i. Internal resistance of ion movement through the electrode ii. Heat loss e. Batteries self-discharge, so over time, they lose their charge 4. The source of battery voltage a. Chemical reactions occur at both electrodes during battery discharge i. @ anode, oxidation occurs, i.e. the loss of electrons ii. @ cathode, reduction occurs, i.e. the gain of electrons b. Both reactions are the half of a single type of reaction k.a. a redox reaction i. red = reduction ii. ox = oxidation  c. The spontaneity with which the reaction occurs is given by the standard reduction potential (SRP) i. The SRP reflects the ease with which an electrode will be reduced and is measured in volts 전자가 얼마나 쉽게 얻어질 수 있는지 (환원) 나타내는 값 ii. The more positive the SRP, the more spontaneously the reaction occurs and thus the higher the reactions voltage 물질이 전자를 얻기 쉬움 (쉽게 환원됨) iii. When applied to a battery, the SRP is… 1. Positive at the cathode (it’s being reduced) reduction = 환원 2. Negative at the anode (it’s being oxidized) a. As at the cathode, the reaction at the anode is spontaneous but in the opposite direction - SRP(환원 전위)는 환원 방향으로의 자발성을 측정함. - anode에서는 산화(oxidation)가 자발적으로 일어나므로, anode의 SRP는 부호가 바뀜. 음수로 나타남. b. So when expressed in terms of SRP, which is for reduction, the voltage is negative c. But when expressed in terms of oxidation, its’ the inverse of the SRP and thus positive d. The total voltage of the redox reaction then is the positive SRP at the cathode + the inverse of the SRP at the anode, which in this case is positive 3. Use the lead acid battery as an example a. At the cathode, the SRP = +1.7 V b. At the anode, the SRP = -0.35 V c. But since the anode is being oxidized and not reduced, the voltage of the reaction at the anode is +0.35 V d. Thus the total redox reaction in a lead acid battery is ~2 V 5. Battery arrangements a. Individual batteries can be arranged in two ways i. Series 1. Anode in one battery is connected to the cathode in another battery  2. Anode in last battery connects back to cathode in first battery across a load ii. Parallel 1. Anode in one battery is connected to anode in another battery 2. Cathode in one battery is connected to cathode in another battery 3. Leads from batteries in parallel are connected to one another across a load b. When batteries are connected in series, battery voltages add i. In the circuit 1. Voltage = the summed voltage of all the batteries 2. Current = the current output from a single battery c. When batteries are connected in parallel, battery charge capacities add i. In the circuit 1. Voltage = the voltage of a single battery 2. Current = the summed current output from all the batteries d. Battery capacity i. Expressed in Amp-hours or Ah 1. Ah = current (C/s) x time = total charge (C) 6. Battery metrics a. Key parameters used to compare batteries i. Voltage ii. Current output iii. Power output iv. Power loss = current output 2 x battery’s internal resistance v. Charge capacity = Ah vi. Energy capacity = Wh = V x Ah  vii. Energy density = Wh/m3 viii. Power density = W/ m3 ix. Specific energy = Wh/kg x. Specific power = W/kg 7. Battery performance a. Batteries need to be charged at a higher voltage than they’re rated at i. Ensures reaction is driven to completion ii. Voltage can’t be too high, however, or battery will ignite b. Battery voltage declines as the battery is discharged i. Due to battery’s internal resistance causing power loss c. Amount available charge from the battery declines as current discharge (I) increases i. Again, due to internal resistance and power loss 1. Remember, the loss goes as I2 2. So, as I increases, loss increases even more d. Available charge also declines with decreasing temperature i. As temperature drops, rate of electrochemical reaction declines 1. This is why keeping batteries in a cooler prolongs battery life; it reduces self discharge 2. Also why it’s difficult to start a car when it’s very cold e. With repeated use, electrodes in battery corrode i. Battery can’t discharge or charge as much as before ii. Discharge/charge cycles reduced in time 8. Hydrogen fuel cells a. Different from batteries in that energy is not stored but provided by a fuel, in this case hydrogen  i. Hydrogen either produced by 1. Electrolysis of water 2. Reformation of methane, i.e., strip the C off of CH 4 a. Results in CO2 emissions b. How fuel cell works i. Hydrogen passed through catalyst that strips off electron, which goes to and out anode ii. Hydrogen ion passes through electrolyte iii. At cathode, hydrogen ion rejoins with electron and O 2 to produce H2O