Tuesday, January 13, 2015

Energy Storage

Last 18th December I attended to one IEEE webinar regarding “Enabling Smart Grids: Energy Storage Technologies Opportunities and Challenges”.
The webinar got an introduction of the different energy storage technologies:

  1. Mechanical: pumped hydro, compressed air (CAES), flywheels
  2. Electromechanical: secondary bateries, flow batteries
  3. Chemical: hydrogen
  4. Electrical: doble layer capacitor, superconducting magnetics (SMES)
  5. Thermal: heat storage, molten salts
This blog tries to introduce the state-of-the-art of the Energy Storage Systems and to expose very interesting example (also energy projects Wiki) and discussions in internet to try to introduce in the topic and show the big possibilities for this emerging technology which is in engineering more time than we know: 

Mechanicals:
Pumped hydro: Pumped-storage hydro electricity (PSH) is a hydroelectric energy storage used by electric power systems for load balancing in the Electric System using gravitational potential energy of water,  pumped from a lower elevation researvoir to a higher elevation during low-cost off-peak electric power, normally during night, depending of the load.


Figure 1_ Picture of Hydroelectric Plant of Susqueda (Northern Barcelona: Hydroelectric Plant which supply to Barcelona City Water and Electricity - See web in Spanish)_Source: Javier Sanchez Rios.


Figure 2_Electricity Demand in real time in Spain during January 5th of 2015. As it has been exposed in the present point, normally the pumped system is working in the low demand, in the case of Spain from 2 to 6 in the night, when the price of electricity and the demand are lower. Source_Ree -Electricity Demand in real time 

Compressed air: Compressed air energy storage (CAES). Small scale systems is normally used as propulsion of mine locomotives. Large scale applications must conserve the heat energy associated with compressed air; dissipating heat there is a decreasing of the performance in the energy efficiency of the storage systems, but for some companies, CAES opens new possibilities in storage systems.

Flywheels: Flywheels is a rotating mechanical device that is used to storage rotational energy. Flywheels have a significant moment of inertia and thus resist changes in rotational speed. Energy is transformed to a flywheel by applying torque to it, thereby increasing its rotational speed, and hence its storage energy.
The common uses of flywheels are:
  • For providing continuous energy when the energy source is discontinuous, in cases of reciprocating engines (piston engine, e.g.; compressor for cooling systems), when the energy source is intermittent, such as torque of the engine.
  • Delivering energy at rates beyond the ability of a continuous energy source. This is achieved by collecting energy in the flywheel over time and then releasing the energy quickly, at rates that exceed the abilities of the energy source.
  • Controlling the orientation of a mechanical system. In such applications, the angular momentum of a flywheel is purposely transferred to a load when energy is transferred to or from the flywheel.
  • For providing stability in the Electric Systems in terms of voltage and frequency, overall, in small or isolated Electric Systems (e.g.: Ree in the Canary Island of Lanzarote)

Electromechanical
Batteries: Composed by Electrodes, Electrolyte and separators.
Secondary batteries: Also called reachargable battery, storage battery, secondary battery or accumulative is a type of electrical battery. Rechargable batteries have a lower total cost of use and environmental impact then disposal batteries. It comprises one or more electrochemical cells because its electrochemical reactions are electrically reversible. Rechargable batteries come in many different shapes and sizes, ranging from button cells to megawat systems connected to stabilize an electrical distribution network. Several different combinations of chemicals are commonly used, including: lead-acid, nickel cadmium (NiCd), nickel metal hydride (NiNH), and lithium ion (Li-ion), and lithium ion polumer (Li-ion polymer) used in second application for grid applications energy storage, most of them coming form Electric Vehicles, enlarging the End-of-life (EOL).


Figure 3_ Nissan Leaf charging Li-ion arranged laminarly batteries during the 2014 Smart City Expo in Barcelona_source: Javier Sanchez Rios

At the same time, it is possible to describe some of the main secondary batteries in use:
Flow batteries:Batteries where the active material is out of the tanks (power and energy independently) and made by Vanadium, being safer batteries.
Sodium (Na) based batteries: Technology which performs at 200ºC by molten salts and sulphur which gives the possibility to work with cheap materials and long life-cycles. The drawback is the corrosive reactions and the consequence of leakage and the cost in maintenace.
Zn-air: Batteries composed by one metal (anode), and O2 from air as cathode which is able to perform at high temperatures. This technology has great maturity and potential for high energy densities, is stable and less dangerous than other technologies.

Chemical: The chemical energy storage is the storage of hydrogen. There are different kind of hydrogen storage; high pressure, cryogenics, and chemical compounds that reversibly H2 upon heating. Underground hydrogen storage is used for grid energy storage for minimizing intermittent in renewable energy sources (solar PV, Wind etc.), as well as providing fuel for fuell cell vehicles (FCV), see example of project in sustainable urban transport by Fuel Cells in Europe and new concept car by hydrogen.
Liquid hydrogen is used in Space Shuttle. However liquid hydrogen requires cryogenic storage (-252,882ºC). Hence, its liquefaction imposes a large energy loss (energy needed to cool it down to the temperature of -252ºC). The tanks must also be well isulated to prevent boil off, but adding isolation increased cost. Liquid hydrogen has less energy density by volume than hydrocarbons fuels such as gasoline by aproximately a factor of four. This highlights the density issue for pure hydrogen: there is actually about 64% more hydrogen in a liter of gasoline (116 grams of hydrogen) than there is in a liter of pure liquid hydrogen (71 grams of hydrogen). The carbon in the gasoline also contributes to the energy of combustion.
On the other hand, compressed hydrogen is stored in a different way. Hydrogen gas has good energy density by weight, but poor energy density by volume versus hydrocarbons, hence it requires a larger tank to store. A larger hydrogen tank will be heavier than the small hydrocarbon tank used to store the same amount of energy. All other factors remaining equal, increasing gas pressure would improve the energy density by volume, making for smaller, but not lighter container tanks. Compressed hydrogen cost 2,1% of the energy content to power to compressor. Higher compression without energy recovery will mean more energy lost to the compression step. Compressed hydrogen storage can exhibit very low permeation.
The applications are increasing but most common: aerospace industry (not only for rocket launches), chemical, automotive, Smart Grids applications (for wind or solar farms) and in new applications like superconducting.


Electrical: Electrical doble-layer capacitor, superconducting magnetics (SMES).
Electrical double-layer capacitors (EDLC), or also called supercapacitors or ultracapacitors, part of a new type of electrochemical capacitors. This kind of capacitors have no the conventional solid dielectric, the capacitance value of an electrochemical capacitor is determined by two storage principels:
  • Double-layer capacitance: electrostatic storage of the electrical energy achieved by separation of charge in a Helmholtz double layer at the interface between the surface of a conduct electrode and an electrolytic solution electrolyte. The separation of charge distance in a double-layer is on the order of a few Angströms (0.3 – 0.8 nm) and is static in origin.

  • Pseudocapacitance: Electrochemical storage of the electrical energy, achieved by redox reactions electrosorption or intercalation on the surface of the electrode by specifically absorbed ions that results in a reversible faradaic charge-transfer on the electrode.
  • Double-layer capacitance and pseudocapacitance both contribute to the total capacitance value of a supercapacitor. However the ratio of the two can vary greatly, depending on the design of the electrodes and the composition of the electrolyte.

Pseudocapacitance can increase the capacitance value by as much as an order of magnitude over that of the double-layer by itself.
Supercapacitors are divided into three families, based on the design of the electrodes:
  • Doble-layer capacitors: with carbon electrodes or derivatives with much higher static double-layer capacitance than the faradaic pseudocapacitance.
  • Pseudocapacitors: with electrodes made of metal oxides or conducting polymers with much higher faradaic pseudocapacitance than the static double-layer capacitance.
  • Hybrid capacitors: capacitors with special electrodes that exhibit both significant double-layer capacitance and pseudocapacitance, such as lithium-ion capacitors.

Supercapacitors have the highest available capacitance values per unit volume and the greatest energy density of all capacitors. They can have capacitance values of 10,000 times that of electrolytic capacitors; up to 12,000 F at working voltages of 1.2 V.
Supercapacitors bridge the gat between capacitors and rechargeable batteries. In terms of specific energy, as well as in terms of specific power, this gap covers several orders of magnitude. However batteries still have about ten times the capacity of supercapacitors. While existing supercapacitors have energy densities that are approximately 10% of a conventional battery, their power density is generally 10 to 100 times as great. This makes charge and discharge cycles of supercapacitors much faster than batteries. Additionally, they will tolerate many more charge and discharge cycles than batteries.
In these electrochemical capacitors, the electrolyte is the conductive connection between the two active electrodes. This distinguishes them from electrolytic capacitors, in which the electrolyte is the cathode and thus forms the second electrode.
Supercapacitors are polarized and must operate with the correct polarity. Polarity is controlled by design with asymmetric electrodes, or, for symmetric electrodes, by a potential applied during manufacture.
Supercapacitors support a broad spectrum of applications for power and energy requirements, including:
  • Long duration low current for memory back up in (SRAMs)
  • Power electronics that require very short, high current, as in the KERS system in Formula 1 cars
  • Recovery of braking energy in vehicles
Supercapacitors have longer life-cycle than electrochemical batteries, which includes lower maintenance and is made by materials easy to recycle.
One example of company which produce supercapacitors is Maxwell

Superconducting magnetics energy storage (SMES): is a energy storage by magnetic field made by the flow of a direct current in a superconducting coil which is cooled by a temperature lower than the critical temperature of superconducting.
The structure is based in three components: one superconducting coil, one power electronics system and one cryogenic cooling system.
Regarding the functionality, once the superconducting coil is under the magnetic field effects, the current is not decreasing, and the magnetic energy is possible to storage indefinitely. The energy storage is able to be managed. For extracting this energy, the current is switched (ON/OFF) quickly by a power electronic control. On the other hand, by the high inductance, the coil behaviour is like a current source which is able to use for loading a capacitor which gives a voltage to a inverter which produces the AC voltage required with only 2-3% of energy losses, being this superconducting with high efficiency.

Thermal: Molten salts
Molten salts: can be employed as a thermal energy storage method to retain thermal energy collected by Solar Power and after used to generate electricity when there is no sunshine or at night.
The molten salts mixtures vary, the most extended mixture contains sodium nitrate, potassium nitrate and calcium nitrate, being those, non-flammable and nontoxic, and has already been used in the chemical and metals industries as a heat-transport fluid.
The salts melts at 131 ºC (268 ºF). It is kept liquid at 288 ºC (550 ºF) in an insulated storage tank. The liquid salt is pumped through panels in a solar collector where the focused sun heats to 566 ºC (1.051 ºF). It is then sent to a hot storage tank. 
This is so well insulated that the thermal energy can be usefully stored for up to a week.


Regarding the opportunities and challenges for energy storage, it is possible to mention the applications in Renewable Energies penetration for reducing or avoiding the “intermittent” issue in this sustainable generation. Such as Renewable Energies needs to get a better maturity, in terms of technology and the consequence in cost-of-energy, energy storage needs as well a certain improvement in technology maturity and cost reduction, for getting better comercial availability, but on the other hand, it needs greater support for developing this technology, from the regulators and institutions, not only economically, also in the awareness for exposing the benefits and value of this technology in the achievement of sustainable global target, knowing the consequence in the energetic plans for being a topic which concern a lot of different technologies (chemical, electrical, thermal) and then, having application in all the technologies which affects energy and its energy management: Energy Efficiency.

From customer point of view, it is necessary to explain the main contributions for customers;
  1. with the possibility for managing the Demand Response, in first term to flatten the peak demand, using the energy storaged for this peaks,
  2. Time-of-use (TOU): giving the possibility to the customer for managing the energy consumption in relation with maximum cost, and then having the possibility to save energy and money to the customer having the needs required at lower cost.
  3. Following with point 2, even giving the possibility to the customer for storage energy when it is at lower cost and selling this energy during high cost, increasing the Return on Investment (ROI).
  4. Possibility for frequency regulation, over all for Transmission System Operators (TSO), but also for customer which requires high frequency regulation for its control systems.
  5. Demand Response: the possibility for having a reserve of capacity to be dispatched when in whatever unexpected event.

Grid Side:
  • Bulk energy service: Arbitrage, storage during low cost periods and sell this energy when increase the price
  • Ancillary services: Frequency regulations: supports system during loss of generation or transmission
  • Support renewable energies: short term intermittency, PV, Wind generation
  • T&D deferral: Use energy storage to support feaders or transformers during avercharge.

The challenges which Energy Storage needs a human-centered approach, having a technical feasability for improving economic revenue in this energy storage investments, at the same time, it is necessary a holistic approach; technically (e.g: batteries: higher energy density), economic, social and environtmentally, considering the safety and energy trends and one worldwide regulation.

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