Energy storage systems (ESS) are growing in importance as renewable energy sources like wind power, wave power and solar are being commissioned and connected to the grid at an ever-increasing rate. Energy Storage Systems are needed to help the grid deal with the instability and unpredictability of such power generation systems and match them to equally variable consumer demand.
In the UK, renewables currently produce over 20% of the country’s electricity and this is likely to exceed 30% by 2020. The current installed capacity is in the region of nine gigawatts indicating the nation’s reliance on more environmentally friendly forms of energy generation into the future.
Battery-based systems are rapidly gaining market share for use as ESSs and gaining acceptance due to advances in their design. Other energy storage approaches can use a range of media, including compressed air, pumped hydro and flywheel. Properly packaged, battery-based systems offer advantages in transportability and size. Advances in battery design and construction have helped manufacturers improve efficiency and lifespan, as well as enhance safety of these systems.
The configuration of batteries and the set-up of the system can bring a variety of benefits to grid quality. Some configurations are useful for rapid response short-term discharge to maintain grid frequency stability and power quality, others meanwhile can supply a longer duration output to perform load balancing and peak shaving, or even backup power on a microgrid.
The technical capabilities and benefits of a battery energy storage system (BESS) can address multiple aspects of power quality and storage.
Frequency regulation - Because utilities must maintain their output within a narrow frequency range, this is a common application for a Power Conversion System (PCS)/BESS. High demand can cause a slight drop in frequency, especially on systems of lower capacity. A BESS can compensate for peak loading with a high-energy discharge through the PCS within a second.
Ramp rate control/capacity firming - This is especially important with renewable energy sources such as wind and solar farms. In these applications, the storage element can fill the gaps that occur when output dips due to a major reduction in wind energy or when clouds move over a solar farm.
VAR support - Reactive loading reduces the efficiency of transmission and distribution lines, but an appropriately designed BESS can compensate by supplying an adjustable range of real or reactive power. This allows more efficient use of power lines and distribution equipment.
Replacing spinning reserve - Reserve capacity helps maintain output during generator failure or unexpected transmission loss, which could require power reductions to customers. Keeping generator capacity online but unloaded wastes fuel and causes unwanted air emissions. A BESS can take the place of conventional spinning reserve generation and improve efficiency.
Black start - This capability allows a power plant to bootstrap itself after a blackout, grid connection loss and/or loss of generation capacity. A BESS can provide the balance of plant power needed for a restart.
Arbitrage/time shifting - This is the storage of low-cost power for later sale at higher prices. Generally this occurs during hours of lower demand.
Transmission and distribution upgrade deferral - Being able to defer additions to T&D infrastructure is attractive to utilities that are experiencing significant, albeit uneven, growth in power usage. Generally, demand is characterized by ever higher peak loads that occur with increasing frequency. Eventually, existing transmission and distribution infrastructure becomes the weak link between a power plant and customers. A utility-scale BESS can be deployed near the load to level out power flow and delay a costly upgrade.
The Battery Energy Storage System
A battery energy storage system consists of two main parts – a bank of batteries and a power conversion system (PCS) used to interface the batteries to the grid. The batteries can be any of a number of available chemistries, with lithium ion being a popular choice in utility scale installations.
Individual battery cells are connected in a series/parallel arrangement in order to obtain the required terminal voltage for highest efficiency and required storage capacity. An integral part of the battery bank is the battery management system (BMS) that monitors battery condition, charge rate and other variables. The BMS will typically ‘report back’ to the PCS or Energy Management System (EMS), allowing it to take action when a battery related anomaly occurs.
The bi-directional PCS is a critical part of the BESS, as it is responsible for the charging and discharging of the batteries, converting their direct current (DC) to the alternating current (AC) used by the grid, and keeping the AC synchronised with the grid frequency.
The PCS uses high-power IGBTs capable of high-speed switching and full-power delivery in either direction within milliseconds. For DC to AC inversion, this pulse-width-modulated (PWM) switching technology includes automatic synchronisation with the AC power grid's frequency and zero crossings. The system can provide automated sequenced shutdown and disconnection in power loss situations, or can be configured to function in island mode, providing backup power for an isolated microgrid.
Other elements in the PCS include devices that monitor operating conditions, detect power quality and provide protection in case of thermal or electrical overload conditions.
Due to the typically harsh or challenging operating environments in which renewable energy infrastructure is usually located, the physical design of the BESS is important to consider. The ability to operate reliably over long periods while exposed to the weather is key. Good thermal management is also vital in order to protect inverters, batteries and ancillary components. The design of cooling systems in grid-tie inverters has traditionally relied on air or liquid water-glycol cooling. Neither is ideal with air cooling having low heat exchange efficiency and consuming significant amounts of energy. Chilled water-glycol requires a substantial volume of liquid to be pumped through the system, which using both space and power; it can also raise concerns about corrosion and other maintenance issues.
An alternative option is closed-loop evaporative cooling. In this system, a refrigerant such as R134a is circulated at high pressure through the thermally critical components inside the PCS. As heat from the components transfers to the refrigerant, it partially evaporates, with the resulting vapor sent to a condenser. The vapor then condenses to a liquid form and returns to the holding reservoir, where it is again pumped through the components. Taking advantage of the heat of vaporisation in a two-phase system has proven to be very efficient, requiring much lower liquid flow than in a water/glycol system, and therefore smaller, lower power pumps.
In addition to simply charging and discharging batteries, many systems provide an important extra benefit – the capability to provide both real and reactive power. This capability provides the ability for the BESS to truly support the grid and to tolerate grid faults. Firmware and programmability in the PCS can allow for flexibility of control and standalone operation. If PCS contains the algorithms for real and reactive power management, it can eliminate or reduce the responsibility of external site management by the utility company bringing many cost, technical and logistical benefits.
ABOUT THE AUTHOR
David Blood is Market Manager EMEA, Energy Grid-Tie Division at Parker Hannifin.
FURTHER INFORMATION
Parker Hannifin, http://www.parker.com/