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:
- Mechanical: pumped hydro, compressed air (CAES), flywheels
- Electromechanical: secondary bateries, flow batteries
- Chemical: hydrogen
- Electrical: doble layer capacitor, superconducting magnetics (SMES)
- 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.
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.
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;
- with the possibility for managing the Demand Response, in first term to flatten the peak demand, using the energy storaged for this peaks,
- 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.
- 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).
- Possibility for frequency regulation, over all for Transmission System Operators (TSO), but also for customer which requires high frequency regulation for its control systems.
- 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.
Bibliography:
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