Recent developments in storage technology and applications

March 3rd, 2015, Published in Articles: Energize

 

Electrical energy storage systems (ESS) are becoming increasingly important in both the renewable energy and the smart grid sectors, as more operators are realising that without ESS it will be difficult to progress forward. Although it was regarded in the past as too expensive, storage is now seen to be an essential economic solution to problems facing the industry. This update examines the latest developments in transportable energy storage systems and their application.

Centralised bulk energy storage has been in use for a long time. Pumped water storage is the most commonly used method and more recently compressed air storage has been piloted. Both suffer from one drawback: they are dependent on geographically suitable sites and must be located where the site is and not where the need is. Latest demands are for storage that can be placed anywhere in the network, which has led to the development of large transportable storage units that can be located anywhere the need is.

The smart grid concept and the proliferation of distributed renewable generation have added to the need for distributed smaller sized storage units, and this has also accelerated development of the technology.

Storage technology applications

Initial development of ESS focussed on backup, transmission line load levelling, and renewable energy applications.It was soon realised that small storage systems are extremely fast and flexible, enabling them to balance fluctuations in the grid much more precisely and efficiently than other generation systems, and the smart storage system was developed to take advantage of these characteristics. Smart storage systems can perform the following functions:

  • Grid support applications
    – Frequency regulation – Demand response
    – Reactive energy provision
    – Transmission and distribution load levelling
  • Price arbitrage: storing electricity when it is cheap and reselling when the price is high
  • Load shifting: shifting loads away from peak by using stored energy.
  • Renewable firming and ramp rate control: Smoothing out the variable output, and helping renewable energy systems provide more consistent power throughout the day.
  • Island or off-grid mode: primary energy storage for off-grid renewable energy systems.

There are several large network storage projects running worldwide, many associated with renewable energy.There is currently no working or trial application in SA using bulk network or on-site grid connected storage and the only bulk storage systems are used in conjunction with off-grid or grid connected renewable energy systems. These systems are relatively small and mainly intended for use by the owner.

Cyclic applications of batteries

The applications listed above place severe operational demands on batteries, and the ability to last under cyclic as well as partial state of charge operation is a crucial measure. The range of operations for ESS may involve extensive periods of operation in partial state of charge (PSoC) mode, as illustrated in Fig 1. Under these conditions the battery undergoes cycles of varying depth and duration at a state below full charge.

Fig.1: Partial state of charge cycling [1].

Fig.1: Partial state of charge cycling [1].


Quantifying this for disparate applications requires a definition of ‘capacity turnover’ that is separate from the concept of a “cycle”. The common understanding of a cycle, which is often quoted in the lifetime of a battery,  is a charging operation followed by a discharging operation, so that a new cycle is marked by a change in direction of power flow into or out of the cell, and not by a particular amount of energy stored and released. The full nameplate capacity of a battery is rarely or never used in a single cycle. Thus, although it is common to count cycles during a test, this is not a measure that can be compared between different tests that may use different application-specific cycles [1].
Capacity turnover measures the total energy throughput of a battery, up to the end of its life, as a multiple of the rated capacity of a battery. A comparison of battery life cycles becomes possible by comparing capacity turnovers.

Technology developments

Interest in transportable (non-fixed) ESS systems is increasing, with a large number of research projects sponsored by government and other agencies in progress worldwide. Used in both on-site and local grid applications, ESS is seen as the vehicle which will enable the move to the next stages of renewable energy penetration and smart grid development.
There are a large number of promising technologies in development stages, but not many have reached commercialisation yet.

Those which are available in the market are summarised below:

Super and ultra-capacitors. Used where rapid charge and discharge capability is required as well as the ability to meet short surges of demand. Characterised by high power density but relatively low storage capacity. These units are finding application in a number of areas, particularly transport, but also in load levelling applications to handle start-up surges, and renewable energy systems where there is a  need to cater for short deep interruptions in output. These devices have a long cycle life compared to electrochemical devices, and are therefore eminently suitable for applications with large numbers of cycles of short duration.The are used mainly for short term storage as they have a high self-discharge rate.

Li-ion batteries. This technology has established itself as the leader in available battery storage devices. Li-on systems are high-power devices. The technology has a very high ratio of power to energy making it particularly suited for grid stabilization applications that often require energy only to be stored for up to a couple of minutes or hours. Available in a wide range of sizes the technology is somewhat limited in maximum size, although this does not prevent paralleling of large numbers of cells. Bulk storage units of up to 30 MW using Li-ion batteries are in service. Li-ion remains the technology of choice for small to medium sized installations, but remains out of reach for smaller systems and domestic users, although there are reports that Tesla and other companies are focussing on this and may be producing a product similar to the automotive units that will be within the reach of the small system owner [2].

Sodium sulphur (NaS) batteries. This high temperature device has higher unit storage capacity than li-ion and is used where large bulk storage is required. Sodium-sulphur batteries have a large storage capacity. These high temperature batteries are well suited to balance the daily fluctuations of wind and sun power. Several units are in operation.

Flow batteries. These units are characterised by high storage capacity but limited energy density per unit. The output power of the unit is limited by the active membrane “cell” which is the heart of the battery, and the circulation rate of the electrolyte. Best suited for a steady output and long term bulk storage. Initially conceived as a large bulk storage system, these systems are now becoming available in smaller compact transportable units in the range of 250 kW/1 MWh. Two types are available:

  • Vanadium redox flow battery: established several decades ago these systems are being used more and more in network and on site storage applications. Vanadium-redox-flow batteries do not self-discharge. That makes them ideally suited to store energy over long periods. Several units in service in South Africa.
  • Zinc-bromine flow batteries: a newcomer in the flow battery field, these units are also available in transportable sizes, and have found use in network and on-site applications.

Hybrid storage systems

A new development in the storage field, these systems consist of combinations of different storage technologies, and are meant to overcome the limitations of individual technologies. No individual storage technology meets all the requirements of the new applications on its own, which may include or relate to:

  • Charge and discharge rates
  • Energy density: The amount of energy stored per unit volume/weight
  • Power density: The power delivery capability per unit, and the ability to deliver short surges of high power.
  • Charge efficiency
  • Average and peak load capabilities
  • Cycle life.
  • Life cycle costs.

The ability to meet short and long term storage demands varies with the technology. Fig. 2 illustrates how characteristics vary with technology.

Fig. 2: Storage capacity vs power density (Author).

Fig. 2: Storage capacity vs power density (Author).

The left hand side starts with ultra-capacitors having high power density and low storage capacity moving across to flow batteries with high storage and low power density and low surge capability.

A typical candidate for a hybrid ESS would be an application that requires economical bulk storage to cater for long term outages, the ability to respond quickly to short term demands, and the ability to provide for very short surges in the network. No technology on its own can cater for this.

Hybrid systems on the market consist of two types:

  • Low types which combine ultra-capacitors to cater for short high demands with Li-ion batteries to cater for long term storage and steady demand situations. The ultra-capacitor handles short cycles and thus also reduces the cycle load on the Li-ion battery. Used typically in an application with a large number of short high power cycles, could be typically load levelling or urban rooftop solar commercial or industrial application.
  • High demand types which combine Li-ion as the front end, with NaS or flow batteries as the long term. The Li-ion battery handles the short term variable and surge demand and bigger battery handles the longer term.

The ultimate hybrid system may combine ultra-capacitors, Li-ion batteries and flow batteries, although no systems of this type are known to be in operation, several are being researched.

System configuration

Units available consist of the appropriately sized individual technologies contained within a single structure. The different technologies cannot simply be connected together but are combined using charge/discharge controllers as shown in Fig 3.

Fig. 3: Hybrid system configuration (Author).

Fig. 3: Hybrid system configuration (Author).

 

Integrated hybrid battery

Ecoult Energy storage systems have developed an integrated hybrid battery [1] containing an ultra-capacitor in its structure.  The technology claims to combine the advantages of proven and dependable lead-acid battery technology with the advantages of an asymmetric supercapacitor, enabling the optimal balance of an energy-storing lead-acid battery with the quick charge acceptance, power discharge and longevity of a supercapacitor.

Its performance exceeds conventional lead-acid cells in PSoC application i.e. operating in a band of charge that is neither totally full nor totally empty leading to viability of use in applications where energy is charged and discharged at significantly higher efficiency. The capacitor integrated into the battery modifies the process associated with the formation and dissolution of sulfate crystals within the negative plate when discharging and charging, respectively. This enables the cell to operate for long periods in the mid-charge band (the most efficient charge/discharge region for lead-acid cells) and, combined with the cycling endurance of the technology, results in an ability to process a much greater amount of energy .

System examples li-ion battery/ultra-capcitor

Maxwell Technologies, has announced that Freqcon, a German developer and distributor of renewable energy systems, has deployed an energy storage system for the Tallaght Smart Grid Testbed in Ireland that uses Maxwell ultra-capacitors and lithium-ion batteries to support grid stability in both residential and industrial settings. Freqcon’s microgrid stabilizer addresses the electricity intermittency challenges that accompany high renewable energy penetration. The testbed uses Freqcon’s microgrid stabilizer for voltage and frequency stabilization, with a combination of lithium-ion batteries and ultracapacitors for active power support in the grid’s distributed network. The Maxwell ultra-capacitors perform fast functions such as frequency response, while the batteries are used for peak shifting and operating reserve [3].

Li-ion Battery/flow battery

Bosch has installed a hybrid Li-ion/ vanadium redox flow battery storage unit to complement the 3 MW wind farm at the small farming community of Baderup in Germany. It is said to have been prompted by a local farmer’s frustration at wind turbines overloading the grid (or maybe cutting out) and causing power cuts during gales on the North Frisian coast.The storage system, installed on a 2500 m2 plot of farmland, has a peak output of 2,3 MW and a total capacity of 3 MWh. It is made up of a 2 MW/2 MWh Sony Li-ion battery store for short-term grid stabilisation and a 325 kW, 1 MWh vanadium redox flow unit, from Vanadis Power of Nuremberg, Germany, for longer-term storage.The vanadium redox flow battery contains 80 000 litres of electrolyte, in the form of vanadium pentoxide dissolved in sulphuric acid [4].

Fig. 4: Integrated hybrid battery [1].

Fig. 4: Integrated hybrid battery [1].

Opportunity

The recent RFI issued by the Department of Energy calling for solutions to the electricity crisis opens the door for privately owned network or on-site storage operated on a feed-in tariff (FIT) basis similar to the original FIT proposal for RE. The idea would be to operate similar to the PWS systems but on a smaller scale, i.e. charge the storage during periods when there is excess generation at low cost, and sell back to the network when there is a shortage, under any of the normal control regimes. The availability of smart storage units makes this possible. The fact that the user will be consuming this energy at a different tariff fits well with the FIT system.

References

[1]    Ecoult: “UltraBattery: Benefits of a Breakthrough Storage Technology” www.ecoult.com/technology/ultrabattery

[2]    D Hull and M Chediak: “Tesla plans battery storage for emerging residential market”, Renewable energy world , 12 February 2015, www.renewableenergyworld.com/rea/news/print/article/2015/02/tesla-plans-battery-storage-for-emerging-residential-market
[3]    Maxwell Technologies: “Ultracapacitors deployed in Ireland microgrid energy storage system”, 18 February 2015, www.prnewswire.com/news-releases/maxwell-technologies-ultracapacitors-deployed-in-ireland-microgrid-energy-storage-system-300037291.html
[4]    J Deign: “Bosch uses hybrid battery system for energy storage”, Energy storage report, 3 December 2014, http://energystoragereport.info/bosch-energy-storage-vanadium-redox-flow

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