Energy Storage Systems

Why it is neededIt permit solar energy to be captured when insolation is highest and then later used when the need is the greatest. So, Energy is stored to use it at a different time than when it was generated.

Make it possible to deliver electrical load power demand during times when insolation is below normal or non-existent.

Improve reliability of solar thermal as well as solar electric system

There are four primary ways to store solar thermal energy:Sensible-heat-storage systems, which store thermal energy in materials (without any change in phase) which having good heat-retention qualities.

Latent-heat-storage systems, which store solar thermal energy in the latent heat of fusion or vaporization of certain materials undergoing a change of phase.

Chemical energy storage, which uses reversible reactions (for example, the dissociation-association reaction of sulfuric acid and water).

Electrical or mechanical storage, particularly through the use of storage batteries (electrical) or compressed air (mechanical) Energy storage.

The materials generally used in sensible heat storage with air as the energy transport mechanism e.g. water, rock, gravel, (or crushed stone), iron shot, Refractory material (magnesium oxide & aluminium oxide) and silicon oxide in a bin has the advantage of providing a large, cheap heat transfer surface. Among of them, Rocks are widely used as:

Rock is more easily contained than water.Rock acts as its own heat exchangerIt can be used for thermal storage at high temperatures, much higher than 100*c The heat transfer coefficients between the air and solid is high.The cost of storage material is low.The conductivity of the bed is low when air flow is not present.

Pebble bed exchanger have good heat transfer characteristics between air and the solids of the bed. This type of storage system has been used in the solar houses or with hot air collector system.

Latent heat is the energy absorbed or released when a substance changes its physical state.

Latent heat is absorbed upon evaporation and released upon condensation to liquid (as in clouds). Latent heat is also absorbed when water melts, and released when it freezes.Materials that undergo a change of phase e.g., Glaubers salt (Na2SO4 . 10 H2O), Fe(NO3 )2. 6 H2O, water, salt Eutectics etc.

Mechanical Energy Storage

Fly WheelsA flywheel is an electromechanical device that couples a motor generator with a rotating mass to store energy for short durations. It is driven by an electric motor during off peak hours stores mechanical (rotational) energy as its speed is increased.

The rotation of flywheel can be used to operate a generator to produce electricity.

The same machine serves as both a motor, when electricity is supplied and as a generator when the armature is rotated by the flywheel.

Mechanical Energy Storage

Fly WheelsPrinciple: Energy is stored in the form of Mechanical Energy.

Flywheels constructed from light weight material (like carbon fiber materials) and magnetic bearings can spin in vacuum at speeds up to 40,000 to 60,000 RPM.

Energy density =0.05MJ/Kg, =0.8

The Energy Density is defined as the Energy per unit mass: Where,V is the circular velocity of the flywheel is the specific strength of a material is the density of the material

Properties of some materials used for building flywheels.

Advantages and Disadvantages

Very compact when compared to other energy storage systems.

Flywheels are used for starting and braking locomotives.

A flywheel is preferred due to light weight and high energy capacity.

It is not economical as it had a limited amount of charge/discharge cycle.

Compressed Air Energy StorageAir in/out

Operation:Uses off-peak electricity to compress air and store it in airtight underground caverns. When the air is released from storage, it expands through a combustion turbine to create electricity.Energy density = 0.2~2 MJ/Kg, =0.5Advantages and disadvantages:Fast start-up.Draw back - Geological structure reliance

Pumped Hydroelectric Energy Storage

Operation:It consists of two large reservoirs located at different elevations.During peak demand, water is released from the upper reservoir. If Production exceeds Demand, water is pumped up and stored in the upper reservoir.Pump used is a Combined Motor and Dynamo.

Advantages and disadvantages:

Most effective with largest capacity of electricity (over 2000MW).Energy density = 0.001MJ/Kg, =0.8Geographical dependence.The capital cost is massive.Soil erosion, land inundation, Silting of dams.

Electrochemical StorageTypes of Batteries:Small CapacitiesLead-Acid BatteriesThey use a chemical reaction to do work on charge and produce a voltage between their output terminals.Energy density is 0.6 MJ/Kg.Efficiency of the cell is only 15%Large Scale

Working of a Lead acid Battery

Fuel CellsDirect conversion EnergyElectricityBurning Fuel? High EfficiencyApplications: E.g.: NASA, Viable alternative to petrol engines.

Types of fuel cells:Classified on the basis of operating conditions and various electrolytes used. Alkaline fuel cells (AFC)Polymer electrolyte membrane (PEM)Phosphoric acid fuel cells (PAFC)Molten carbonate fuel cells (MCFC)Solid oxide fuel cells (SOFC)Regenerative fuel cells

Energy densities of some energy storage methods.

Advantages:No green house gasesNot much political dependenceMore operating time.Disadvantages:Storage of Hydrogen due to highly inflammable nature of H2. Though metal hydrides(FeTiH1.7) and NH3 can be alternative.High capital cost due to Platinum catalyst used in the process.

Electromagnetic energy storage is also called as super conducting energy storage

It is defined as: It is possible to store electric energy as a magnetic energy, even when the electric current is flowing into the coil of the superconductive cable, since the electric current generates continuously a uniform magnetic energy without attenuating

Magnetic Energy Storage

Super Conductors

SMES systems store energy in a magnetic field created by the flow of direct current in a coil of superconducting material that has been cryogenically cooled.

Principle: At low-temperatures, electric currents encounter almost no resistance.

Stores energy in the magnetic field.

Environmental friendly and Highly efficient.

Super CapacitorsUse of thin film polymers for the dielectric layer Carbon nanotubes and polymers are practical for super capacitors In future - carbon nanotubes with ceramics Reduce the effect of fluctuations Longer life time which reduces maintenance costs.

Which is better???Comparing one method of energy storage with another is pointless.

The reason - None of them are optimal for all purposes.

Different storage methods differ in capacity and maximum usable storage time.

For large scale storage Underground thermal, pumped hydro and compressed air energy storage systems are preferable.

Superconductors can store energy with negligible losses.

Fuel cells are a viable alternative to petrol engines due to their high efficiency.

Flywheels have a narrow range and are not an answer for large scale operations.

Conclusion:Reliable and affordable energy storage is a prerequisite for using renewable energy.Energy storage therefore has a pivotal role in the future.Energy storage is the most promising technology currently available to meet the ever increasing demand for energy.

Limitations of solar thermal energyLarge area is required to collect solar thermal energy.Solar energy is not available during night and clouds.Direction of rays changes continously with time.Energy storage is essential.High CostSolar central power plant in MW range are not economical