Part of the BRE Trust

Steve Pester Smart Solar NSC 2015

Solar Storage Technologies

Overview of next few minutes…

– Challenges

– Some solutions

– Types of storage

– Main battery technologies

– How batteries behave (esp Li-ion)

– Factors for technology selection

– Power diverters

– Further info

Main challenges

– Peaks and troughs in demand

– predictable, but quite large – expensive peak capacity

– Grid resilience & security

– Variability of renewable sources

– Predictable in short term, but not controllable

PV generation potential – daily sun path

Source: PVSyst

Solar resource versus national demand -

A typical day in winter

Source: BRE

30

GW

15

10

5

20

25

Domestic buildings

Non-Domestic buildings

Does not include industrial processes, street lighting, agriculture, etc

Overall daily demand

Source: Gridwatch, 27-01-15

The worst – football & royal weddings!

The solutions

– Demand side management

– International interconnectivity (Supergrids)

– Storage

o Building-level o Grid-level

Storage offers: o Spinning reserve o Peak shaving o Load shifting o Voltage & frequency

stabilisation

Image: Bine Informationsdienst

Types of non-fossil storage

– Electromagnetic / electrostatic

• Supercapacitors • Superconducting magnets

– Heat

• Ground o Heat pumps: inter-seasonal? o Geothermal

• Water o Domestic hot water o Underground tanks

• Heat Engines

– Mechanical

• Pumped Water • Flywheels • Compressed air

– Chemical

– Batteries • Static • Vehicle batteries for

static use • Many chemistries

– Hydrogen • Electricity via fuel cells • Heat via fuel cells • Heat by combustion • Make methanol • Possible by

photosynthesis?

Main battery types of interest at present

Type Pro’s & Cons Maturity

Lead-acid (Pb-acid)

• Cheap • Can deep cycle VRLA types • Limited cycle life • Vented last longer than VRLA • Low energy density (~40Wh/kg)

Proven

Nickel-Cadmium (NiCd)

• Higher energy density than Pb-acid • Improved cycle & calendar lifetime over

Pb-acid • Memory effect • Toxicity of Cd - banned in EU for many

applications now

Proven

Nickel-metal-hydride (NiMH)

• Largely replaced NiCds for small portable applications

• Have been used in electric cars • Poor self-discharge characteristics • Being superseded by Li-ion

Proven

Types (cont)

Type Pro’s & Cons Maturity

Various Sodium chemistries (sulphur, metal hydride, nickel chloride)

• High temperature (300C) • Grid-level • Energy density 3-5 x Pb acid • Overall good reliability & service life • 1 infamous fire in Japan

GWh installed, but continue to develop

Redox Flow • Liquid electrolytes flow over membrane exchanging electrons

• Decouples capacity and power • More from Green Acorn later…

Types (cont)

Type Pro’s & Cons Maturity

Lithium-ion (Li-ion)

• Fastest developing battery technology because of EV market

• Costly, but large price reductions imminent • Possibility of using 2nd hand EV batteries • ~200Wh/kg energy density • ~5000 - 10000 cycle life • Low self-discharge • Most promising in short term for renewables • Care and safety precautions required!

Thermal run-away & fire under fault conditions

• Several different chemistries

Widely used in EVs, but still developing

There are many others!

Mistreated Li-ion batteries

http://www.bbc.co.uk/news/business-25733142

How batteries behave

X kWh X kWh

Charge Discharge

Not quite…

Round trip efficiency

X * Ec kWh Y * Ed kWh

Charge Discharge

Charge losses

Discharge losses

To make matters worse… losses are time-dependent

– Faster discharge = lower efficiency

– Rate of 1C = charge / discharge full capacity in 1 hour, e.g. 0.2C discharge = full discharge in 5 hours

Lead-acid discharge curves

Li-ion cycle lifetime v. depth of discharge

100

1000

10000

100000

1000000

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Cycles

Depth of discharge per cycle

Lifetime taken to be 70% capacity remaining

Thermal stresses affect performance & lifetime

– Temperature

• Hot or cold

• Lead-acid affected more than Li-ion

• All chemical reactions slower at low temperature, so charge / discharge reduced, but shelf life extended

Balloon analogy

Pressure

Volume Voltage

State of charge

Self-discharge – balloons are porous

…so are batteries

Limited charge / discharge rates

Internal resistance affects voltage & current seen at the terminals

X kW

y kWh

Key factors for technology selection

– Understanding the above factors is essential for correct design

– Key characteristics to match to application:

• Power

• Capacity

• Energy density • Cycle life

• Self-discharge rate

– Technology development status

• Reliability • Safety – check safety record and handling/installation precautions

– Manufacturer bankability

Power diverters

PV

Solar power always

supplies house &

appliances first

Diverter

The diverter senses any surplus & directs to

the immersion heater, until the set water

temperature is reached.

Without a diverter, if there is

surplus solar power, it goes out

to the electricity grid

How diverters interface to building

Cable from meter

Consumer unit

Existing house-

hold circuits

PV inverter

Energy

Diverter

Immersion circuit

2 pole isolator

Immersion

heater

Hot

water

cylinder

CT sensor

around L or N

only

L

N

E

Loads for diverters

– DHW cylinder is the “standard” load

– Thermal stores

– Battery chargers

– Towel rails

– Storage heaters – better with wind turbines

– U/floor heating - supply/demand mismatch with PV

– Some diverters have secondary load output

Diverters - Points to note

– No combi-boilers

– Power delivered to load must match that available from PV

– Watch out for interference generated (EMI)!

– Common switching approaches

• Phase angle control

• Burst mode control

• Pulse-width modulation

– Ask to see EMC report (CE)

• EN 61000-3-2 (harmonic current emissions) • EN 61000-3-3 (voltage fluctuation and flicker)

– Issues with heat pump compatibility

Where to get further info

– IET Code of Practice to be published April

– Training on the CoP will be available

– A Good Practice Guide on Electrical Storage, EA Technology

For more information contact:

Steve Pester

NSC (Watford Office)

pesters@bre.co.uk

Mob: 07528 976224

Office: 01923 664 729