Solar Storage Technologies Steve Pester Part of the BRE Trust Smart Solar NSC 2015
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 GW Domestic buildings 30 25 NonDomestic buildings 20 15 10 5 Source: BRE 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 (Pbacid) 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-metalhydride (NiMH) Largely replaced NiCds for small portable applications Have been used in electric cars Poor self-discharge characteristics Being superseded by Li-ion Proven Cheap Can deep cycle VRLA types Limited cycle life Vented last longer than VRLA Low energy density (~40Wh/kg)
Types (cont) Type Pro s & Cons Maturity Various Sodium chemistries (sulphur, metal hydride, nickel chloride) GWh installed, but continue to develop Redox Flow Liquid electrolytes flow over membrane exchanging electrons Decouples capacity and power More from Green Acorn later High temperature (300 C) Grid-level Energy density 3-5 x Pb acid Overall good reliability & service life 1 infamous fire in Japan
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 Discharge Charge X kwh X kwh Not quite
Round trip efficiency Charge X * Ec kwh Charge losses Discharge Y * Ed kwh Discharge losses
To make matters worse losses are time-dependent Lead-acid discharge curves Faster discharge = lower efficiency Rate of 1C = charge / discharge full capacity in 1 hour, e.g. 0.2C discharge = full discharge in 5 hours
Li-ion cycle lifetime v. depth of discharge Lifetime taken to be 70% capacity remaining Cycles 1000000 100000 10000 1000 100 10% 20% 30% 40% 50% 60% 70% 80% Depth of discharge per cycle 90% 100%
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 Voltage Volume State of charge
Self-discharge balloons are porous so are batteries
Limited charge / discharge rates y kwh X kw Internal resistance affects voltage & current seen at the terminals
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 CT sensor around L or N only PV inverter Consumer unit N L Existing household circuits E Cable from meter Immersion circuit Energy Diverter Hot water cylinder 2 pole isolator Immersion heater
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