Part of the BRE Trust Steve Pester Smart Solar NSC 2015 Solar Storage Technologies
Jul 16, 2015
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• Have been used in electric cars • Poor self-discharge characteristics • Being superseded by Li-ion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)
Mob: 07528 976224
Office: 01923 664 729