Designers of uninterruptible power supplies, buses, cars and the alternative energy community have been seeking ways of storing electrical energy efficiently and cost effectively.
Traditionally, the energy storage market has been dominated by lead acid batteries, which are inexpensive and efficient but have their own limitations. The advent of advanced batteries such as lithium-ion and ultra capacitor energy storage solutions has offered a breakthrough in efficiency and cost-effectiveness.
Energy Storage Solutions
There are many available methods for energy storage, such as kinetic energy in flywheels, as static charge, chemically, heat, and as potential energy in water dams (pumped storage). The efficient and cost-effective conversion of electrical energy into stored energy is the major challenge.
The stored energy needs to be transformed to provide the required current and voltage as demanded by the particular load. For this purpose, batteries and ultra capacitors are an ideal option due to their ability to store energy in the direct current (DC) form. When coupled to power electronics, the conversion of DC energy to the alternating current (AC) - which most loads require - takes place.
Lead acid battery designs have historically dominated battery technology and have undergone significant advances in recent decades, especially in Valve Regulated Sealed Lead Acid (VRLA) designs. As a result, the cost of these batteries has become lower, making it the ideal option for most uninterruptible power supply and other static and dynamic energy storage applications.
Relatively new developments like thin plate pure lead designs have improved energy density and power delivery capability, making it the technology of choice for many short discharge time applications, especially for 1-15min. In spite of all of the developments in lead acid battery technology and its applicability in various fields, it has its own disadvantages:
- Weight is a drawback as lead is a very heavy element
- Reliability problems for higher voltage strings involves many cells in series. Cell failure affects performance and causes damage to the entire string if not detected and rectified quickly
- Relatively short real application lifetimes, usually 3-10 years. Application lifetimes are further reduced in high temperature environments
- High maintenance costs because regular manual checking of batteries, or even automated checking, considerably increases cost of ownership
- Limited deep cycle performance capability
- Environmental issues with lead and associated recycling costs
In spite of all of these shortcomings, VRLA batteries will remain the preferred technology for most commodity static storage applications, for instance UPSs, in the near future, as it continues to offer low initial capital costs.
The commercialization of cost-effective electric cars has driven large-scale investments in advanced battery designs. Although there are many potential techniques, the lithium-ion technology solutions have become an attractive option owing to their lightweight, good acceptance and high rate discharge performance, good deep cycle performance, and comparatively high tolerance to elevated temperatures.
However, cost and safety are the major challenges to be addressed. Fortunately, cost and safety performance continue to improve, as vehicle manufacturers require lithium-ion batteries in large quantities. Lithium-ion batteries have also become an attractive technology for challenging static and dynamic energy storage applications. Flow batteries are another attractive battery technology, where energy storage takes place in electrolyte tanks and energy passes through cells for charging and discharging.
There are applications that demand high power storage in the sub one minute autonomy range, for which ultra capacitors are the ideal option as they apply static charge more directly instead of storing energy chemically. Very high peak power capability and round trip efficiency, which means minimal power loss in the charge/discharge cycle, are the advantages of ultra capacitors.
Ultra capacitors in modular format are now available with necessary voltage balancing and protection circuitry, such as the LS Mtron ultra capacitor module installed in PCS100 UPS-I Industrial UPSs (Figure 1).
Figure 1. LS Mtron ultra capacitor module installed in PCS100 UPS-I Industrial UPSs.
Reduced Autonomy Times
UPSs have to deal with the progressively changing loads and requirements over time. Larger scale UPSs are mainly used in data centers and so the definition of an acceptable battery autonomy time has been changed radically. Previously, data center operators demanded autonomy times at UPS full load operation of 30min to 1h.
However, actual autonomy times were much higher due to that fact that actual data center loads were much lower than this and hold up required real power loading on the batteries. This would enable operators to detect the issue, analyze and then shut down servers on time. However, data center operators demand much shorter autonomies nowadays due to the following reasons:
- If back-up diesel generators fail to start on the first or second attempt, they are unlikely to be started within any acceptable autonomy time.
- Impact of unplanned power outages has been reduced by applications such as cloud computing and redundant computing configurations.
- Demand for reduction of battery size continues to increase due to increasing space constraints and cost pressures .
- Users have clear knowledge about the actual costs of battery ownership and maintenance and larger battery systems have a higher ongoing cost of ownership.
- Many tier three and four data centers feature dual redundant reticulated uninterruptible power, with each UPS loaded at considerably less than half loading (typically around 25%). This makes extended full load autonomy specifications unnecessary.
Based on these factors, the selection of an appropriate battery for efficient supply of high levels of power with a low internal resistance can save cost and space. Although this requirement can be met with VRLA pure lead batteries, they have their own limitations. Much better service life, power density and tolerance to increased ambient temperature are the advantages of advanced lithium-ion batteries, but they have a higher capital cost. However, the improved temperature capability enables substantial savings in air-conditioning capital and ongoing costs.
Very large mega data centers or large industrial applications, for instance semiconductor fabrication plants, are closely coupled to the high voltage transmission grid with high reliability. Very short outages, such as switching changeovers between redundant feds or very deep voltage sags due to weather events or network faults are the most common problems.
Autonomies in the order of seconds are useful in these cases and the technology of choice is the reliable and power-dense ultra capacitor technology. ABB has supplied several hundreds of megawatts of ultra capacitor backed industrial grade UPSs (Figure 2) with full load autonomies of 2-3s. There is an upward trend towards this technology.
Figure 2. A PCS100 UPS-I installation with ultra capacitor storage in a semiconductor facility.
Electricity grid support applications increasingly require battery energy storage systems (BESS). Electrical power can be supplied back after storage to the grid whenever required to support renewables, support grid frequency and offset peak demand or shift demand from day to night. The PCS100 Energy Storrage System converters from ABB have been applied to interface the AC electicity grid to DC Lithium-Ion battery strings.
Figure 3. PCS100 ESS successfully applied with a zinc bromide flow battery at Redflow in Australia.
Besides shifting real and reactive power to and from the grid whenever required, ABB’s PCS100 ESS product have advanced features that make the power electronic grid interface to behave like a virtual generator. The system is capable of supporting the grid when connected and even determining grid loss. It can continue to support any local loads in island mode after disconnecting from the grid, paving the way for advanced micro-grid development, which includes integration of UPS functionality.
The combination of both UPS function and grid support function is required in many applications. Some users prefer UPSs for supporting the grid with load shedding features, and in some grid support applications that require a level of UPS function. These requirements can now be met with the developments in convertor technology in conjunction with sophisticated energy storage technologies.
The combination of next-generation energy storage technologies and the advances in power electronic converters enables significant developments in renewable power integration, power protection and grid support.
About ABB Power Conditioning
ABB is a leading supplier of UPS and power conditioning products – ABB's PCS100 portfolio is a unique line up of low and medium voltage power conversion technology. The PCS100 power converters demonstrate highly reliable and cost-effective performance. With the PCS100 product portfolio, ABB have the power conversion technology for every need. Starting from a few kVA to many MVA and a wide range of supply voltages. ABB has installed more than 900 MW of power conditioning solutions worldwide. It’s business as usual with ABB Power Protection
This information has been sourced, reviewed and adapted from materials provided by ABB Power Conditioning - Discrete Automation and Motion Division.
For more information on this source, please visit ABB Power Conditioning - Discrete Automation and Motion Division.