ENERGY STORAGE SYSTEMS

1. KONGUNADU COLLEGE OF ENGINEERING AND TECHNOLOGY (AUTONOMOUS) 20EE701OE – ENERGY STORAGE SYSTEMS T.Muthukumar, Assistant Professor/BME, Kongunadu College of Engineering and Technology.

2. UNIT I – ENERGY STORAGE TECHNOLOGIES Introduction - Need of energy storage - Battery - Components of cells and batteries - Classification - Operation of a cell - Theoretical cell voltage, capacity and energy - Electrochemical principles and reactions: Cell polarization - Electrical double-layer capacity and ionic adsorption - Mass transport to the electrode surface - Factors affecting battery performance - Standards

3. Introduction • The Conversion of excess electricity into a different form of energy which can be reconverted into electricity with minimum losses. • This can be done to reduce the gap in supply and demand of electricity and renewable energy sources Types of Energy storages • Storage in the Fuel Distribution System • Periodic Storage • Long-Term, or Seasonal, Storage • Daily and Weekly Storage C) Portable Applications That Require Energy Storage

4. Two types are, 1. Storage Methods for Use with Portable Electronic Devices. 2. Energy Use and Storage in Vehicles Types Based on Energy Type: Electrical, Mechanical, Chemical and Thermal

5. Types Based on the applications: Low power applications in remote areas, • Mainly to supply transducers and emergency terminals. • Medium power applications in remote areas, • Such as individual electrical systems and Power supply to villages. • Power-quality applications. • Network connection application with peak leveling. • Small scale systems-Stored as kinetic & chemical energy- Compressed air, hydrogen, supercapacitors and superconductors. • Large scale systems, stored as gravitational energy in hydraulic systems, thermal energy, chemical energy in batteries or compressed air.

6. NEED FOR ENERGY STORAGE • Energy storage has many benefits. It is particularly important for the development and integration of renewable energy technologies • Some renewable energy sources have intermittent generation, which means that electricity is only produced when the sun is shining or when the wind is blowing. • This creates supply and demand discrepancies because consumers may still require electricity when renewable sources are not producing. • Currently, grids deliver electricity in real-time, meaning electricity is being consistently produced to meet consumer demand.

8. As a result, electricity generation systems are built to meet peak demand (the hours when most electricity consumption occurs, for example during the afternoon of a hot summer day when everyone is operating their air conditioning). • Energy storage enables a lower-cost generating source to produce electricity at a different point in time to be stored and then used to meet times of peak demand. • Energy storage is also commonly used to smooth out the minor fluctuations in energy output for small and large electricity generation sources. • Storage also provides increased reliability and strengthens system resilience at large and small substation levels. • Energy storage is also commonly used in transport, like in electric vehicles, trains and bikes.

9. Parameters of Storage Technologies Storage capacity: defined as the amount of energy available in the storage device after completing the charging cycle Energy available: determined by the dimensions of the generator-motor system used in the conversion process of stored energy Discharge time: Total energy stored/Max peak power τ(s):Discharge time, in s Wst: Total energy stored, in Wh Pmax: Maximum or peak power, in W Efficiency: evaluated from the ratio between the energy released and the stored energy. η: Efficiency of storage technology

10. Wut: Useful or recoverable energy for a given point of operation, in Wh Durability: given by the number of times that the storage device can release energy, from the level for which it was designed. It is expressed as the maximum number of cycles, N, each one corresponding to a charge and discharge processes. Autonomy: Refers to the maximum time that the system can continuously release energy. a: Autonomy, in s Pdt: Maximum power discharge, in W

11. BATTERY • Solid State Batteries: a range of electrochemical storage solutions, including advanced chemistry batteries and capacitors • Flow Batteries: batteries where the energy is stored directly in the electrolyte solution enabling longer charge/discharge cycles, usually four hours each. • Flywheels: mechanical devices that harness rotational energy to deliver instantaneous electricity.

13. Compressed Air: utilize compressed air to create energy reserves • Pumped hydro-power: creates energy reserves by using gravity and the manipulation of water elevation • Thermal: capturing heat or cold to create energy

16. COMPONENTS OF CELLS AND BATTERIES • A battery is a device that converts the chemical energy contained in its active materials directly into electric energy by means of an electrochemical oxidation-reduction (redox) reaction. • The cell consists of three major components: 1. The anode or negative electrode -The reducing or fuel electrode-which gives up electrons to the external circuit and is oxidized during the electrochemical reaction.

17. 2. The cathode or positive electrode-The oxidizing electrode-which accepts electrons from the external circuit and is reduced during the electrochemical reaction. 3. The electrolyte-the ionic conductor - which provides the medium for transfer of charge, as ions, inside the cell between the anode and cathode. • The electrolyte is typically a liquid, such as water or other solvents, with dissolved salts, acids, or alkalis to impart ionic conductivity • The cell itself can be built in many shapes and configurations—cylindrical, button, flat, and prismatic-and the cell components are designed to accommodate the particular cell shape

18. CLASSIFICATION OF CELLS AND BATTERIES Electrochemical cells and batteries are identified as primary (non-rechargeable) or secondary (rechargeable), depending on their capability of being electrically recharged. Primary Cells or Batteries • These batteries are not capable of being easily or effectively recharged electrically and, hence, are discharged once and discarded • Many primary cells in which the electrolyte is contained by an absorbent or separator material (there is no free or liquid electrolyte) are termed ‘‘dry cells.’’

19. Applications: • Lightweight source of packaged power for portable electronic and electric devices • Lighting • Photographic equipment • Toys • Memory backup • Large high capacity • Primary batteries are used in military applications, signaling, standby power • Host of other applications

20. Advantages: • Good shelf life • High energy density at low to moderate discharge rates • Inexpensive • Giving freedom from utility power • Ease of use.

21. Secondary or Rechargeable Cells or Batteries • These batteries can be recharged electrically, after discharge • They are storage devices for electric energy and are known also as ‘‘storage batteries’’ or ‘‘accumulators Applications: The applications of secondary batteries fall into two main categories: • Those applications in which the secondary battery is used as an energy-storage device, generally being electrically connected to and charged by a prime energy source and delivering its energy to the load on demand. Examples are automotive and aircraft systems, emergency no-fail and standby (UPS) power sources, hybrid electric vehicles and stationary energy storage (SES) systems for electric utility load leveling

22. 2. Those applications in which the secondary battery is used or discharged essentially as a primary battery, but recharged after use rather than being discarded. Example, in portable consumer electronics, power tools, electric vehicles, etc., for cost savings (as they can be recharged rather than replaced),and in applications requiring power drains beyond the capability of primary batteries. • Features of Secondary batteries • High power density • High discharge rate • Flat discharge curves • Good low-temperature performance • Energy densities of secondary batteries are generally lower than those of primary batteries

23. Reserve Batteries • In this type, key component is separated from the rest of the battery prior to activation. • The reserve battery design is used to meet extremely long or environmentally severe storage requirements that cannot be met with an ‘‘active’’ battery designed for the same performance characteristics Applications: • Deliver high power for relatively short periods of time, in missiles, torpedoes, and other weapon systems.

24. Fuel Cells • Fuel cells, like batteries, are electrochemical galvanic cells that convert chemical energy directly into electrical energy. • Fuel cells are similar to batteries except that the active materials are not an integral part of the device (as in a battery), but are fed into the fuel cell from an external source when power is desired. Fuel cell technology can be classified into two categories 1. Direct systems where fuels, such as hydrogen, methanol and hydrazine, can react directly in the fuel cell 2. Indirect systems in which the fuel, such as natural gas or other fossil fuel, is first converted by reforming to a hydrogen-rich gas which is then fed into the fuel cell

25. Applications • Use of the hydrogen/oxygen fuel cell, using cryogenic fuels, in space vehicles for over 40 years. • Use of the fuel cell in terrestrial applications • Utility power • Load leveling • Dispersed or on-site electric generators • Electric vehicles.

26. OPERATION OF A CELL Discharge When the cell is connected to an external load, electrons flow from the anode, which is oxidized, through the external load to the cathode, where the electrons are accepted and the cathode material is reduced. Discharge reaction Anode material - Metal Cathode material - Chlorine (Cl2)

27. Negative electrode: anodic reaction (oxidation, loss of electrons) Zn → Zn2+ + 2e Positive electrode: cathodic reaction (reduction, gain of electrons) Cl2 + 2e → 2Cl- Overall reaction (discharge): Zn + Cl2→ Zn2+ + 2Cl-(ZnCl2 )

28. Discharging of a cell

29. Charge During the recharge of a rechargeable or storage cell, the current flow is reversed and oxidation takes place at the positive electrode and reduction at the negative electrode. In the example of the Zn/Cl2 cell, the reaction on charge can be written as follows: Negative electrode: cathodic reaction (reduction, gain of electrons) Zn2+ + 2e → Zn Positive electrode: anodic reaction (oxidation, loss of electrons) 2Cl-→ Cl2 + 2e Overall reaction (charge): Zn2+ + 2Cl → Zn + Cl2

30. Charging of a cell

31. THEORETICAL CELL VOLTAGE, CAPACITY AND ENERGY • The theoretical voltage and capacity of a cell are a function of the anode and cathode materials Free Energy • Whenever a reaction occurs, there is a decrease in the free energy of the system, which is expressed as

32. Theoretical Voltage The standard potential of the cell is determined by the type of active materials contained in the cell Anode (oxidation potential) + cathode (reduction potential) = standard cell potential. For example, in the reaction Zn + Cl2 → ZnCl2

33. Theoretical Capacity (Coulombic) The theoretical capacity of a cell is determined by the amount of active materials in the cell. It is defined in terms of coulombs or ampere-hours the theoretical capacity of the Zn/Cl2 cell is 0.394 Ah/g Theoretical Energy The capacity of a cell can also consider on an energy (watthour) basis by taking both the voltage and the quantity of electricity into consideration

34. ELECTROCHEMICAL PRINCIPLES AND REACTIONS Batteries and fuel cells are electrochemical devices which convert chemical energy into electrical energy by electrochemical oxidation and reduction reactions, which occur at the electrodes Cell Polarization A cell consists of an anode where oxidation takes place during discharge, a cathode where reduction takes place, and an electrolyte which conducts the electrons within the cell (1) Activation polarization, which drives the electrochemical reaction at the electrode surface. (2) Concentration polarization, which arises from the concentration differences of the reactants and products at the electrode surface and in the bulk as a result of mass transfer.

35. These polarization effects consume part of the energy, which is given off as waste heat, and thus not all of the theoretically available energy stored in electrodes is fully converted into useful electrical energy.

36. Cell polarization (1)Activation polarization, which drives the electrochemical reaction at the electrode surface. (2)Concentration polarization, which arises from the concentration differences of the reactants and products at the electrode surface and in the bulk as a result of mass transfer.

37. Electrical double-layer capacity and ionic adsorption • When an electrode (metal surface) is immersed in an electrolyte, the electronic charge on the metal attracts ions of opposite charge and orients the solvent dipoles. There exist a layer of charge in the metal and a layer of charge in the electrolyte. This charge separation establishes what is commonly known as the ‘‘electrical double layer’’ • The electrical double-layer affect is manifest in the phenomenon named ‘‘electrocapillarity’’. There exist thermodynamic relationships that relate interfacial surface tension between electrode and electrolyte solution to the structure of the double layer.

38. Mass transport to the electrode surface • Mass transport to or from an electrode can occur by three processes: • Convection and stirring • Electrical migration in an electric potential gradient, • Diffusion in a concentration gradient. • The first of these processes can be handled relatively easily both mathematically and experimentally. If stirring is required, flow systems can be established, while if complete stagnation is an experimental necessity, this can also be imposed by careful design. • The migration component of mass transport can also be handled experimentally and described mathematically, provided certain parameters such as transport number or migration current are known. • Migration of electro active species in an electric potential gradient can be reduced to near zero by addition of an excess of inert ‘‘supporting electrolyte,’’ which effectively reduces the potential gradient to zero and thus eliminates the electric field which produces migration. • Enhancement of migration is more difficult. This requires that the electric field be increased so that movement of charged species is increased. • The third process, diffusion in a concentration gradient, is the most important of the three processes and is the one which typically is dominant in mass transport in batteries.

39. Battery discharge characteristics-voltage levels

40. Effect of temperature on battery capacity

41. Battery service life at various discharge loads and temperatures