Electrical Energy Storage (EES)
Till about a few years ago, we thought that electricity cannot be stored and needs to be consumed as and when it is generated. Times are changing; today electricity can be stored in megawatt scale thanks to developments made in storage technologies and solutions. These electricity energy storage (EES) applications are increasingly becoming viable around the world.
The smart grids are expected to be the biggest achievement of the 21st century! And energy storage technologies are going to be an important part of it.
The EES technologies around the world
Energy storage technologies encompass a large set of diverse technologies. They are broadly classified into mechanical, electrochemical, chemical, electrical and thermal energy storage systems as shown in the figure below.
Globally there is about 202 GW of grid connected storage systems of which 135 GW is pumped hydro and 65 GW is UPS systems, rest about 2677MW being new storage technologies:
1. Pumped storage hydro – the most successful energy storage systems due to their fast response and storage capacity, pumped storage hydro have been proven to be excellent reserves. The world installed capacity is about 135GW. Conventionally, two water reservoirs at different elevations are used to pump water during off peak hours from the lower to the upper reservoir (charging) and the water flows back to move a turbine and generate electricity (discharging) when required. Their long lifetimes and stability are what makes them ideal storage systems. However technical and commercial issues have prevented their large scale adoption.
2. Compressed air energy storage (CAES) – 440MW of installations exist around the world. This technology is based on the conventional gas turbines and stores energy by compressing air in an underground storage cavern. Electricity is used to compress air and when needed the compressed air is mixed with natural gas, burned and expanded in a modified gas turbine. The turbine produces the same amount of output power as conventional gas turbines but uses only 40% of the gas. Round trip efficiencies of up to 70% are reached. Undersea insulated airbags based systems are the latest under trials. The advantage of CAES is its large capacity; disadvantages are low round-trip efficiency and geographic limitation of locations.
3. Flywheels – rotational energy is stored in a large rotational cylinder where the energy is maintained by keeping its speed constant. A transmission device is used to accelerate or decelerate the flywheel by supplying and extracting electricity. When the speed is increased higher amounts of energy are stored. A vacuum chamber is used to reduce friction, and the rotors are made of carbon fibre composites suspended by magnetic bearings. Flywheels are extensively used for space applications. Latest generation flywheels are reported to be suitable for grid applications. Beacon Power commissioned a 20MW FES plant in New York in 2011 with storage capabilities of up to 5 MWh in 15 mins. Experiments with wind farms in California have also shown encouraging results. Total installations of 42MW exist in the world. The long life of this technology with relatively less maintenance requirements and excellent cycle stability make it an ideal storage solution, however the high levels of self-discharge due to air resistance and bearing losses make it less efficient.
4. Batteries - originally invented by Italian scientist Alessandro Volta in early 1800s – batteries were essentially zinc and copper electrodes in sulphuric acid. Those days there was no need for electricity! In 1859, lead acid battery emerged as a power source for automobiles. In 1888 alkaline batteries were invented. Next major breakthrough was in 1970s with nickel- cadmium cells that set off the electronic boom, walkman's and power tools became popular inventions. The next major turning point was 1991 with the commercialisation of Lithium batteries (LIB) which actually revolutionised the portable electronic gadgets industry. Nickel-Metal Hydride (Ni-MH) batteries emerged in 1997 in Toyota and Honda cars while mass production of LIBs in Korea started in the 2000s, mass production of LIBs by LG Chemicals and Hyundai for Electric Vehicles have been in production since 2009.
Batteries have distinctly different market segments: Appliances - electric and electronic devices; EVs - two wheelers and 4 wheelers; home applications and MW scale grid integrated applications. Requirement for each of these domains being very different, the size, capabilities and technologies can be different. So, different technologies can and will co-exist in battery technologies. However all of them need to mature to higher efficiencies and capacities. The various battery technologies available are:
NaS - by far sodium sulphur batteries are considered the most matured technology. Main manufacturer is NGK Insulators, Japan. About 300MW of installed capacity exist around the world. However fire in the NGK factory in Sep 2011 put a strong question mark on the future of NaS. Despite that in December 2012 a 1MW NaS battery park was installed in Berlin by Yunicos in association with Vattenfall for frequency regulation which flattens fluctuations within milliseconds. Yunicos installs battery parks with 20 years guarantees. In the Web to energy project they recently put on line the LIBs to communicate on the 61850 protocol. Closer to home, at Mitsui House in Delhi they have recently completed a microgrid with NaS battery.
LIB - rechargeable Lithium Bromide has changed our daily life. This technology has products for all the four industry segments- electronics, EVs, home applications and grid connected applications. Li is a comparatively light metal and is very active. Developments in material sciences have allowed for improvements in their density and the advanced LIBs under development are targeting 300wh/kg and 1$/Wh which will be non-flammable. Next generation by 2020 is expected to reach 700wh/kg and 50 cents/Wh which may be fully solid state. Current generation numbers are 140wh/kg and 3.2 $/Wh. Without prejudice to the great leaps made by LIBs the recent Dream Liners troubles have put doubts on LIBs scalability to megawatt levels. Heat dissipation might warrant active cooling. That is a whole lot expensive as temperature sensors and cooling systems are going to be expensive.
Lead Acid - Lead acid batteries are the world's most widely used battery type and have been commercially deployed since about 1890. Their usability decreases when high power is discharged and this makes them unviable as MW scale solutions.
Vanadium redox - this technology is a flow type battery that is emerging as a dark horse. Flow batteries use PEM fuel cell technology. UTC Labs in Hartford, Connecticut have a 20 kw flow battery with vanadium redox. UTC is ready for commercialisation of this technology and intends to manufacture it in India considering the huge market potential here.
5. Electric Double layer capacitors - EDLC are super/ultra capacitors that were invented by GE in 1957. Standard oil developed it in the 1960s and sold it to NEC who commercialised it in 1970s as super capacitors to provide backup for computer memory. Later used in space applications, aircrafts etc, these EES have very long life and can withstand lakhs of cycles.
6. Super conducting magnetic energy storage - very much in its infancy stage, it has a superconducting coil and a cryogenically cooled refrigeration system that once charged stores the energy in the magnetic field created in the coil for an indefinite period of time. 1MWh systems used for grid applications, 20 MWh systems than can provide 40MW for 30 mins or 10MW for 2 hrs are under development.
7. Thermal storage – systems use cold water, hot water or ice storage to store the heat and
use for later. The efficiencies vary with the material. They are
important for integrating large scale renewable energy as concentrted
solar thermal technology can be used as a reliable and despatchable
source of energy to balance the supply and demand.
Market projections are compelling - 238 billion in next ten years in US alone. The biggest drivers are regulatory interventions. The various Energy storage associations are:
2009- California Energy Storage Association CESA
2010 -Texas Energy Storage Alliance TESA
2011 - ESA Advocacy council
2012 - India Energy Storage Association IESA
2013-China Energy Storage Association, German Energy Storage Association (BVES), European Association for the Storage of Energy EASE! And a Global Energy Storage Alliance is mooted!
With the planned integration of nearly 41GW of Renewable Energy capacity in India by the end of 2016-17, there is a need for storage applications to address the issues of variability, unpredictability and location dependency of RE sources.