Storage: the heartbeat of renewables

The importance of electrical energy storage

By Jewel Thomas

As many countries try to increase the share of REs (renewable energies) in their electricity generation portfolio, a major issue facing utilities is EES (electrical energy storage). IEC International Standards for EES technologies aim to provide a safe and stable energy supply and to integrate electricity from intermittent RE sources into the overall distribution grid. IEC created TC (Technical Committee) 120: EES (Electrical Energy Storage) Systems, in 2012 to develop International Standards on systems aspects of EES.

Shiobara 900 MW pumped-storage plant (Courtesy of Tokyo Electric Power Co.)
Shiobara 900 MW pumped-storage plant (Courtesy of Tokyo Electric Power Co.)

Old and new

Pumped-storage hydropower currently represents the largest and most flexible EES solution and is experiencing significant growth. Modern battery systems and chemistries have improved, increasing the capabilities of such systems. Flywheels can capture energy from RE sources in a mechanical form and can deliver uninterrupted power to the grid nearly instantly when needed. Other effective EES systems are thermal energy storage for capturing excess energy from thermal solar plants during peak insolation periods, usually in molten salts, to release it during dark hours. Chemical storage, in the form of hydrogen or SNG (synthetic natural gas) produced from excess electricity offers more storage opportunities.

Batteries still central to future grid storage

A new generation of advanced safe, low-cost and efficient enough batteries should play a major role in the future global EES landscape and in grid management. The global market for these batteries, which include Li-ion (lithium ion), sodium metal halide, NaS (sodium sulphur), advanced lead-acid and flow batteries, is expected to grow from USD 182,3 million in 2014 to USD 9,4 billion in 2023. However, this introduction is still limited to high-value applications like frequency regulation and demand charge mitigation.

IEC TC 21: Secondary cells and batteries, and its SCs, prepare International Standards for all types of rechargeable cells and batteries installed in EES systems.

The first ever grid-connected HBr (hydrogen bromine) flow battery storage solution for use with renewables was connected to a test site in Israel in April 2014.

Energy harvesting

Interest is growing significantly for energy harvesting or energy scavenging, the process associated with the collection of low-grade energy from sources such as ambient or waste heat, human power, solar, thermal and kinetic energy, and their conversion into electrical energy. Viewed initially mainly as a convenient way of powering sensors, small wireless electronic devices and low-power systems, opportunities are now also opening up for energy harvesting use in larger applications.

Energy harvesting is widely used for powering sensors and actuators, such as those found in certain types of MEMS (Micro-Electro-Mechanical Systems), which are increasingly deployed in sectors such as automotive and medical. IEC International Standards for MEMS are prepared by IEC TC 47: Semiconductor devices, and are tested by IECQ (IEC Quality Assessment System for Electronic Components).

The urban public transport sector offers a great potential for energy harvesting and a more energy-efficient transportation sector. For example regenerative charge braking and energy harvesting shock absorbers are being fitted to buses to charge batteries and supercapacitors for providing extra power. In some countries, energy harvesting pavements have also been installed in certain heavy pedestrian traffic locations, such as train stations or office buildings, for powering energy-efficient lights or other systems.

The super storage capacity of supercapacitors

Supercapacitors most commonly EDLCs (Electrochemical Double Layer Capacitors) have very favourable characteristics in terms of power density. They are also resistant to shock and vibration.and have the ability to be charged and discharged countless times without any degradation in performance. This is in stark contrast to chemical batteries which have a defined life span in terms of cycling.

Supercapacitors may appear as serious competition for batteries, in particular with Li-ion technology, but should more likely be considered as a complementary technology. IEC TC 40: Capacitors and resistors for electronic equipment, has published International Standards for EDLCs, and has earmarked these and hybrid EDLCs, which combine a capacitor and a battery, as being in need of appropriate standardization. In addition to uses in the transport sector, supercapacitors are found in cordless power tools, computers and consumer electronics. They are also found in wind turbine blade pitch systems, particularly offshore where their long life and reliability is a key advantage.

However, some of the disadvantages of supercapacitors include a low energy density (ranging from around 1 Wh/kg to 30 Wh/kg), particularly when compared with Li-ion batteries (about five times more energy density); and chemical batteries in terms of discharge curve. Although falling rapidly, costs for supercapacitors are still relatively high due to the increased difficulty in creating advanced materials like graphene.

Nevertheless, supercapacitors are rapidly becoming a multi-billion dollar market.

Piezoelectrics power onwards

Although the first practical piezoelectric devices emerged little more than three decades ago they are becoming increasingly commonplace and can now be found in a diverse array of devices and applications. With new materials and designs constantly emerging, developments in piezoelectric technology focus on achieving better operational characteristics as well as on improving environmental performance.

Given their suitability as electromechanical transducers, these materials are used in numerous sensor applications such as those found in the ultrasonic measurement of distance in air, materials-testing equipment, accelerometers, pressure sensors, and in medical equipment. These materials are also employed in spark generators, such as those used in an electronic ignition cigarette lighter.

Developing needed International Standards

Within IEC, most International Standards for piezoelectric technology, with the exception of those for piezoelectric transducers, which are prepared by IEC TC 29: Electroacoustics, and IEC TC 87: Ultrasonics, are developed by IEC TC 49: Piezoelectric, dielectric and electrostatic devices and associated materials for frequency control, selection and detection.

Better for the environment

One key area of piezoelectric materials development is focused on new applications and materials to improve sensitivity, durability and operational performance. Some of the new materials being considered for piezoelectric ceramics are lead-free ones, to address toxicity problems and potential challenges associated with final disposal.

Shiobara 900 MW pumped-storage plant (Courtesy of Tokyo Electric Power Co.) Shiobara 900 MW pumped-storage plant (Courtesy of Tokyo Electric Power Co.)
A123 systems nanophosphate EXT™ grid storage solution (Photo: A123 systems) A123 systems nanophosphate EXT™ grid storage solution (Photo: A123 systems)
This 50 kW HBr flow battery system provides up to 100 KWh of energy (Photo: EnStorage Inc.) This 50 kW HBr flow battery system provides up to 100 KWh of energy (Photo: EnStorage Inc.)