Prof. R. Shanthini 09 Feb 2013 Energy Storage Energy is stored to use it at a different time than...

Prof. R. Shanthini 09 Feb 2013

Energy StorageEnergy Storage

Energy is stored to use it at a different time than when it was generated.

The process of converting the energy to storable form means that some energy is lost.

Additional energy is lost when the energy is released or recovered.

Ideally, storage is avoided to have a more efficient process.

Renewable energy, however, is often intermittent (like wind and sun), and storage allows use at a convenient time.


Types of Energy StorageTypes of Energy Storage

Electricity can be stored by converting it into another form such as potential, kinetic or chemical energy.

Electrical energy storage technologies include the following types of storage media:

- Battery electric storage system (BESS)- Flow batteries- Fuel cells- Flywheel energy storage (FES)- Superconducting magnetic energy storage (SMES)- Super capacitor energy storage (SCES)- Compressed air energy storage (CAES)- Pumped hydro energy storage (PHES)

Energy and Buildings 42 (2010) 2124–2130

Prof. R. Shanthini 09 Feb 2013 www.consumersenergy.com/content.aspx?id=1830

Electricity is produced from this 27-billion gallon reservoir using six turbines that drive electric generators. The same turbines double as giant water pumps to fill the reservoir with water from Lake Michigan.

Real-life example: 1872 MW generating capacity

Pumped Hydro Energy Storage (PHES)Pumped Hydro Energy Storage (PHES)



Reversible pump-turbine can spin an alternator to generate electricity.

It also can act as a pump to pump water uphill using electricity.

This reversibility allows excess electrical energy to be used to pump water to a higher storage reservoir to be used as an energy source later.

PHES is the most widespread high-energy storage technique.

PHES provides rapid response (<1 minute) to support intermittent renewable generation.



http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html

The energy used to pump a water volume (V) to a height (h) with a specific pumping efficiency (ηp) is given by:

Epumping = · g · h · V

ηp

Overall efficiency of the energy storage system

= Egenerator / Epumping

The energy supplied to the electrical network by a generator of efficiency (ηg) can be obtained by:

Egenerator = · g · h · V · ηg



http://sun.anu.edu.au/pumpedhydro



http://www.forumforthefuture.org/greenfutures/articles/irelands-3-billion-euro-bid-energy-independence

Ireland plan: A €3.45 billion project will comprise a 2GW peak power plant, including 18 onshore wind farms, a hydro station and a grid transmission connection.




Hybrid wind-hydro power plant


Compressed Air Energy Storage (CAES)Compressed Air Energy Storage (CAES)

Off-peak electricity is used compressed air, which is stored. During peak hours, compressed air is used to generate electricity. This system operates in the same way as a conventional gas turbine, but compression and expansion operations are independent and they occur at different times.


Conventional gas turbine (recalling)Conventional gas turbine (recalling)

Comp-ressor

fresh air

Combustion Chamber

fuel

GasTurbine

gasesto the stack

Gen

compressed air

hot gases



The above shown CAES system that operates with the excess generation of a wind farm.




Air is compressed and stored in reservoirs, aquifers or underground cavities.

The stored energy is released during periods of peak demand, expanding the air through a turbine.

When air is compressed for storage, its temperature will increase according to


kk

P

PTT

)1(

1

212

where T is the temperature, P is the pressure, k is the polytropic index of the expansion, and subscripts 1 and 2 indicate the initial and final state of compression, respectively.



The heat generated can be retained in the compressed air or in another heat storage medium.

This way, heat can be returned to the air before its expansion in the turbine.

This method is called adiabatic storage system and achieves a high efficiency.

However, if the heat generated in compression is lost, additional heat must be supplied to the air by burning a fuel.

In general, for a given pressure ratio, the work of the turbine is directly proportional to the absolute temperature of admission.



Flywheel Energy Storage (FES)Flywheel Energy Storage (FES)

Conventional flywheel


Flywheels store electric power as kinetic energy of the rotor mass spinning at very high speeds, according to


2

2

1 IE

where is the inertia momentum of the flywheel and is the angular speed.

I




The charging/discharging of the device is carried out through an integrated electrical machine operating

- either as a motor to accelerate the rotor up to the required high speeds by absorbing power from the electric grid (charge mode)

- or as a generator to produce electrical power on demand using the energy stored in the flywheel mass by decelerating the rotor (discharge mode).



The system has very low rotational losses due to the use of magnetic bearings which prevent the contact between the stationary and rotating parts, thus decreasing the friction.

In addition, because the system operates in vacuum, the aerodynamic resistance of the rotor is outstandingly reduced.

These features permit the system to reach efficiencies higher than 80%.



Flywheels have the ability to charge and discharge rapidly, and are almost immune to temperature fluctuations.

They take up relatively little space, have lower maintenance requirements than batteries, and have a long life span.

Flywheel devices are relatively tolerant of abuse, i.e. the lifetime of a flywheel system will not be shortened by a deep discharge unlike a battery.



The stored energy is directly proportional to the flywheel rotor momentum and the square of the angular momentum, a reason why increments in the rotation speed yield large benefits on the storage energy density.

2

2

1 IE


Battery Electric Storage System (BESS)Battery Electric Storage System (BESS)

- have high energy densities- technology is matured - relatively easy to use

Let us consider the following four types:

- Lead-acid

- Nickel cadmium (NiCd)

- Sodium sulphur (NaS)

- Nickel metal hydride (NiMH)

- Lithium ion (Li-ion)


Lead-acid (PbA) BatteriesLead-acid (PbA) Batteries

Cathode: Lead peroxide (PbO2) on a lead (Pb) lattice support

Anode: Sponge lead on a lead lattice support

Electrolyte: Sulphuric acid and water

Energy Conversion and Management 58 (2012) 134–148


Anode (oxidation): e2PbSO)(SO)(Pb 4

24 aqs

Cathode (reduction):

)(OH2PbSOe24)(SO)(PbO 24242 lHaqs




http://gicl.cs.drexel.edu/wiki-data/images/e/e6/Screenshot.png


The lead-acid battery is the oldest and most mature technology that has been used for electrical energy storage.

They are ideal for small-cycle renewable energy integration applications.

They can be discharged repeatedly by as much as 80% of their capacity and hence are suited for grid connected systems where users sell power back to the grid through net metering.




They provide a cost-competitive and proven solution to a range of storage requirements owing to

- low investment costs- lowest self-discharge of all rechargeable battery

systems - relatively ease of maintenance

Drawbacks: Drawbacks: - limited cycle life - poor performance at low and high ambient temperatures - failure due to deep and continuous cycling - environmentally unfriendly lead content and acid electrolyte

which result in a large eco-footprint




Nickel cadmium (NiCd) BatteriesNickel cadmium (NiCd) Batteries

Cathode: Nickel oxide hydroxide (Nickel hydroxy oxide) on a nickel foam, graphite or iron substance

Anode: Metallic cadmium pressed onto a nickel wire mesh

Electrolyte: Potassium hydroxide (KOH)




Cathode and anode are rolled up and placed in a steel case. The cathode and anode are separated by a porous poly-meric separator strip (three thin layers that consist of nylon/poly-propylene/nylon), with the electrolyte free to diffuse through it.




The chemical reactions during discharge are: At the cadmium electrode: At the nickel electrode:

The net reaction during discharge is

During recharge, the reactions go from right to left.

The alkaline electrolyte (commonly KOH) is not consumed in this reaction and therefore its specific gravity, unlike in lead–acid batteries, is not a guide to its state of charge.




Nickel cadmium batteries are a robust and proven alternative to lead-acid batteries and rank alongside them in terms of their maturity.

Longer cycle life, higher energy densities and low maintenance requirements highlight superiority of NiCd batteries in comparison with lead-acid batteries.

NiCd batteries offer many advantages in PV applications and factors such as their cycling ability, durability, long life and reliability make them ideal for operating under adverse conditions.




Drawbacks:Drawbacks:

- contain toxic heavy metals

- suffer from severe self-discharge


Sodium sulphur (NaS) BatteriesSodium sulphur (NaS) Batteries

Cathode: molten sulfur

Anode: molten sodium

Electrolyte: solid beta-alumina.

Beta-alumina solid electrolyte (BASE) is a fast ion conductor material used as a membrane in several types of molten salt electrochemical cell. Currently there is no known substitute available.



During discharge, sodium ions produced at the negative electrode are transported through the porous separator to the liquid sulphur positive electrode, reducing some of the sulphur to polysulphide ions.



If a load is connected to terminals, electric power is discharged through the load.

During the discharge, sodium ions converted from sodium in a negative electrode pass through solid electrolyte then reach to sulfur in positive electrode.

The electrons finally flow to outside circuits.

The electric power is generated by such current flow.

With the progress of the discharge, sodium polysulfide is formed in positive electrode; on the contrary, sodium in negative electrode will decrease by consumption.



For the battery to operate effectively it is necessary to maintain its temperature at about 300 to 350oC and this requires the use of sophisticated methods of construction for the battery pack, together with auxiliary heating during charging and periods of non-use.

It is essential that the battery is not allowed to cool below 200oC, as the sodium electrolyte freezes solid at this temperature.

Reactivation then becomes slow and difficult and the freezing of the sodium can damage the battery because of the mechanical stresses introduced.



The sodium-sulphur battery has a high-energy density of 110 Wh/kg, a high power density of 150 W/kg and is capable of 1000 cycles of deep discharge in use.

Drawback: Sodium polysulfides are highly corrosive.



During the charge, the electric power supplied from outside form sodium in negative electrode and sulfur in positive electrode by following the reverse process of the discharge.Because of this, the energy is stored in the battery.


Nickel metal hydride (NiMH) BatteriesNickel metal hydride (NiMH) Batteries

Cathode: Nickel hydroxyl oxide on a nickel foam substrate

Anode: Mischmetal (Mm) hydrides on a nickel or a nickel plated steal mesh

Electrolyte: Potasium hydroxide

22 H)2/1(OHe2MmOH

charged in the right direction of the equation above and discharged in the left direction.

Mischmetals (Mm) is the name given for a group of metals from lanthanide series (or rare earths), ytterium, titanium, zirconium, nickel and vanadium.

(stored as Mm-Hx)



Nickel metal hydride (NiMH) BatteriesNickel metal hydride (NiMH) Batteries

Used in electric vehicles


Lithium ion (Li-ion) BatteriesLithium ion (Li-ion) Batteries


They have a large ratio of power to weight and are used in cell phones and laptops.

They achieve energy storage efficiencies of close to 100%

Anode: Graphite or lithium salt of titanium oxide (TiO)

Cathode: Metal oxide such as CoO2

Electrolyte: Lithium salt is an organic solvent


Lithium ion (Li-ion) BatteriesLithium ion (Li-ion) Batteries


Drawbacks: - high investment costs - Lithium metal is very reactive and explosive



http://www.greentechmedia.com/research/report/grid-scale-energy-storage-technologies-and-forecasts-through-2015

Prof. R. Shanthini 09 Feb 2013 Energy Storage Energy is stored to use it at a different time than...

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