Energy storage in Renewable Energy P.C.Pant Scientist Ministry of New and Renewable Energy pcpant@nic.in

Nature of renewable energy supplies and real challenge Renewable energy supplies are continuing or repetitive current of energy occurring in natural environment The initial input power from renewable energy supplies therefore is outside our control; and matching supply and demand in the time domain is a real challenge Two ways to overcome this challenge are : Either match the load to the availability of renewable energy supply when it is available, or for constant load, hybridize it with an auxiliary energy supply system Storing the energy for future use i.e. keeping it available when and where it is required

Methods of energy storage Chemical : hydrogen, ammonia Heat : hot water, steam, molten salt Electric : Capacitors, electromagnets Gravitational : water reservoir Mechanical : Flywheel Photosynthesis: Biomass Electro chemical: Batteries, fuel cell?

Normal applications of battery Regular deep cycling (as in consumer devices, electric vehicle) Standby use (normally kept at full charge so as to use in case of an emergency such as telephone exchanges) Starting, lighting and ignition (for road vehicles) In all such operation battery is normally given full charge (adequate overcharge) after discharging.

Condition in RE systems Limited (variable)amount of charging energy available from PV array Therefore no guarantee that battery will be fully charged at the end of the day Normally shallow cycle operation Autonomy reserve for certain days is generally specified Even in specifying autonomy, there is certain charge left in the battery (~20%)

Main functions of a battery in RE systems Energy Time shift Energy intensive Supply capacity Voltage support Power intensive Congestion relief Load following Frequency regulation Power inensive Power quality

Understanding battery Storage capacity: not fixed, depends on discharge rate and temperature Never try to extract full capacity (DOD)(restrict upto 80%) More daily DOD, less cycle life Battery voltage during charging depends on amount of current being fed, higher the charging current, faster will be the voltage gain, but does not mean that all the current is absorbed Temperature dependence of cycle life

Charging profile of Li ion battery

Capacity at cut-off voltage Capacity with full saturation Typical charge characteristics of lithium-ion. Adding full saturation at the set voltage boosts the capacity by about 10 percent but adds stress due to high voltage. Charge V/cell Capacity at cut-off voltage Charge time Capacity with full saturation 3.80 3.90 4.00 4.10 4.20 60% 70% 75% 80% 85% 120 min 135 min 150 min 165 min 180 min 65% 76% 82% 87% 100%

Battery research at SEC Facility available: life cycle network, reserve capacity network, temperature cycling Presently manufacturers can avail the facility to get their product evaluated for its capacity, charge and energy efficiencies, reserve capacity, high rate discharge, temperature effect on their product, cycle life etc. Facility is being upgraded to evaluate batteries as per IEC, BIS and JIS specifications

Project implemented Design development of suitable batteries for PV applications with CECRI Evaluation of various types and brands of batteries in actual field conditions Development of suitable charge controllers, MPPT for PV applications Development of Indian standards for batteries in SPV application

Way Ahead Introducing energy storage in large size power projects specifically in islands and isolated places Identifying suitability of different types of batteries for specific applications Extensive R&D both on development and applications Working on other energy storage components specifically on Supercapacitor Developing suitable standards, test methods and test facilities.

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