energy storage pt 1

What is the main motivation behind energy storage?

renewable energy is inflexible and variable which means a lot of this energy is wasted

CfD

Contract for difference - buyer pays the difference between current value of an assest and the value at initiation of contract

grid parity

the point at which the cost of generating electricity from renewable sources is equal to or lower than the cost of purchasing electricity from traditional fossil fuel sources.

Base load negatives

plants with a high CAPEX must run continuously to be economical. This means their output can’t easily be adjusted and energy is wasted.

capture value

the ratio between average value of electricity produced by wind and the average value of electricity overall

how can electtricity systems be made flexible?

adjust electricity generation to match demand  (using G.I.E.S.)

Use electricity storage

adjust demand to match supply

Install over-capacity and simply use turn-down.

Use interconnectors to spread energy over different regions which have different demand profiles and wheather patterns

GIES

Generation integrated energy storage - store energy in convenient form before converting to electricity

net demand

total electricity demand - renewable input

measures demand that must be met from flexible resources or storage

examples of short duration storage

flywheels

super-capacitors

batteries

examples of medium duration storage

thermo mechanical technologies

examples of long duration storage

synthetic and bio - fuels

Exergy

useful energy - maximum useful work that can be obtained from a system at a certain state. (not conserved).

what two quantities are associated with energy storage>

How much energy can it discharge in a single cycle and

How much energy can it discharge over its whole lifetime. 

why is energy storage expensive?

the containment is typically 500 times the cost of energy it contains

KPIs for energy storage

cost per unit energy storage

cost per unit power conversion

effective turnaround efficiency

time for power ramp up/down

energy stored over invested

longterm additional loss per unit time stored

maintenance/replacement costs

additional loss from power conversion

system benefits

slew rate

the maximum rate at which an electronic signal can change - crucial for maintaining signal integrity in electronic circuits.

capacity/utilisation factor

ratio of average power output compared to peak power output

Capacitors

2 conductive plates with dielectric material inbetween. Energy is stored in dielectric material

Advantages of capacitors

You can charge-discharge them many times and they still work.

The energy in a capacitor can be released extremely quickly - high power

Many have an extremely low dissipation-factor … often as low as 0.1% - 0.2%.

A capacitor can hold its energy for a very long time with negligible loss.

Inductor

stores electrical energy in static magnetic field when current passes through. Use iron core with small air gap where most of the energy is stored

disadvantages of inductors

Very poor overall energy density.

Inductors wound with copper (or other metal) wire are losing energy all the time through resistive loss in the wire.

superconductors

utilize superconducting coils to store energy in the form of a magnetic field - negligible resistance but very expensive

Balanced 3 phase power

all three phase voltages (and currents) have equal magnitude, are 120° apart, and supply equal power to the load. This results in smooth power delivery and efficient operation of equipment.

Timescales for grid storage

inductance - prevents rapid changes in voltage <5ms

inertia - 5ms-1s

fast control - 1s-20s

inductance and inertia diminishing with renewable energy use

inertia time constant

spinning kinetic energy/rated load

poisson’s ratio

the ratio of strain perpendicular to load to strain in direction of applied load in a material under uniaxial loading. Matrix can be used to relate biaxial stress and strain

principle stresses

shifting axes to find only normal stresses

Uniaxial case

1 principle stress in 1 direction

pure shear case

opposite principle stresses - stretching in one direction and compression in the other direction

Equal principle stress case

either stretching or compression with equal load in both directions simultaneously

SED

strain energy density - strain energy per unit volume

what does max energy storage in spring depend on?

max SED of material

volume of spring material

utilisation of spring material

examples of springs used for energy storage

suspension

decelerators for trains

rapid release of mechanical energy - cross bow

flywheel

a rotating mechanical device that stores energy as kinetic energy by spinning at high speed. It smooths out fluctuations in power, provides short bursts of energy when needed, and resists sudden changes in rotational speed due to its inertia.

flywheel inertia

J - sum of mr² points around circumference of flywheel

peripheral speed

linear speed of a point on the rim of a flywheel as it rotates. It represents how fast the outer edge is moving (v=ohmr)

how are forces balanced in a thin rimmed flywheel

centrifugal force pushes out and a component of tension in the wheel prevents the whell from breaking apart

why are wavy spokes used in flywheels

to provide flexibility to absorb thermal expansion, reduce stress concentrations, and allow slight radial movement of the rim. This lowers peak stresses, improves fatigue life, and helps the flywheel stay balanced at high speeds.

how can you optimise flywheel

select a highest max stress*volume with lowest cost

high energy per unit mass maximise stress/density

high energy per unit volume maximise stress

disadvantages of gravitational potential stored energy

limited by strength and weight of material used to lift weight

high capital cost

long build times

limited locations

advantages of pumped hydro

Typically have high power and storage capacities

Lose little to no energy over time

Efficiency typically 75%-85%

Operational lives in >50 years

what are the two types of gravitational stored energy?

Type 1:  the same weight is always attached to the power-conversion system.

Type 2:  where the weight “passes-through” the power-conversion system

pumped hydro

uses two water reservoirs at different elevations to store energy. Excess electricity pumps water uphill (storing potential energy), and when power is needed, the water flows back down through turbines to generate electricity.

Components of comopressed air energy storage system

compressor/expander

thermal store

compressed air store

What happens to temperature when air is compressed

Increases

adiabatic

no heat transfer

Why is multistage compression done?

cooling between stages is a trade off between equipment cost and power input

prevents temperature from getting too high

what is total heat taken out from isothermal compression equal to

total work done

How can work returned by compressed air systems be improved

Pre-heat air before expansion to keep the pressure higher and produce more work. Isothermal expansion keeps P high throughout which is ideal scenrio but difficult to achieve

compressed air turnaround efficiency

electricity out / electrical energy + heat exergy input

How are work equations adjusted for expansion

change r (pressure ratio) for r^-1