The ability of wind and solar PV to meet instantaneous electricity demand is limited by the fact that they are intermittent generators – dependent on the instantaneous weather conditions at the site of their installation. For these clean, low-cost generators to provide significant contributions to total energy use on an electricity grid, they must be deployed in conjunction with an energy storage technology.
Energy storage “absorbs” fluctuations in demand and generation to ensure that supply meets demand at all times. One ubiquitous and flexible energy storage technology is the common battery. Secondary batteries (i.e. rechargeable batteries) are suitable for integration into electricity networks of all sizes, and most locations.
Distributed small-scale battery banks can contribute to the least-cost operation of Australian electricity networks by reducing demand from over-loaded feeders during peak usage periods. They can also assist solar PV owners to reduce their dependence on fossil fuel-fired grid electricity by storing excess solar generation for later use.
Larger battery systems can provide ancillary services to wholesale electricity markets, supporting grid frequency following sudden losses in generation or increases in demand. They can also provide ramping control for wind and solar power plant output, as well as for industrial loads. For these industrial customers, demand charges can be reduced by behind-the-meter batteries dispatched by smart controls during peak demand.
In remote and off-grid areas, where high-cost electricity is provided by diesel generators, battery systems can allow for the integration of higher amounts of low-cost electricity from wind and solar PV.
A range of battery technologies and chemistries exist, with the benefits, limitations, and applications discussed in the subsequent sections.
Introduction to Terminology
Batteries absorb electrical power and deliver it on demand at a later time. They do this by converting DC electrical energy to chemical potential energy during charging, and vice-versa during discharging.
Power is the rate at which energy is delivered. Conversely, energy is power delivered over time. For example, a theoretical battery rated to 3 kW/6 kWh can deliver up to 3 kW of power at any time, and based on an energy capacity of 6 kWh could deliver this power for two hours. Alternatively, the battery could provide a lower level of power for longer (i.e. 1 kW for six hours, 0.5 kW for 12 hrs etc.). Real battery behaviour is more complex than the hypothetical description above, but for now it is sufficient to explain the difference between energy and power.
To describe the more complex behaviour that is displayed in practice, a battery is often thought of as a constant voltage source in series with an impedance. Increasing the current drawn from the battery decreases the source voltage due to the voltage drop across this impedance. The magnitude of the impedance is referred to as the internal resistance and helps to understand the voltage response of a battery under load.
Because battery capacity depends on the rate at which it is discharged, the hypothetical 3 kW/6 kWh battery described above is an unrealistic model for some battery technologies. However, other battery technologies perform better at high discharge rates, showing less capacity reduction with increasing discharge rates. These batteries are often best-suited to high power/low energy applications.
In referring to battery capacity, ampere-hours (Ah) are more frequently used than kWh. The Ah capacity can be multiplied by the nominal voltage to get the approximate capacity of the battery system in Wh (e.g. a 48 V battery system with capacity of 100 Ah should deliver 4.8 kWh of energy). The Ah capacity is often specified at a given C-rate, which indicates the length of time over which the battery is discharged. For example, the battery capacity of 100Ah described above might apply when the battery is discharged slowly at C120 (over 120 hrs), but fall to 67 Ah when the battery is discharged at C5. Thus, the C-rate should always be specified alongside the Ah capacity.
Introduction to Working Principles
In most cases, a battery consists of an anode, a cathode and an electrolyte. The anode and cathode are electrically conductive plates that are covered in active materials and immersed in the electrolyte – a substance that separates into ions (positively and negatively charged molecules) – when dissolved in a solvent.
Ions from the electrolyte react at the anode to generate free electrons (making it the negative electrode). Conversely, reactions at the cathode require electrons to proceed (making it the positive electrode). When the circuit between the anode and cathode is closed (via a load such as a fan, heater, motor etc.), free electrons from the anode travel through the load via the external circuit to the cathode. The potential energy (voltage) of the electrons becomes mechanical energy at the load, and in this way electricity does mechanical work.
As the battery discharges the build-up of reaction compounds on the anode and cathode reduces the area available for further reactions. Similarly, as the ions in the electrolyte are being consumed in the creation of the compounds, the concentration of the electrolyte is reducing until it resembles only the original solvent. One or both of these processes at their completion make a battery “flat”. Charging a battery reverses this process, converting electrical energy into stored chemical energy and returning the battery (ideally) to its original state.
With batteries being electrochemical in nature – relying on chemical reactions to release and store energy – they are sensitive to temperature and their capacity to store energy and deliver power will degrade with both time and use. Higher charging and discharging rates increase losses and reduce battery lifespans. Similarly, higher depths of discharge (DOD) tend to result in faster capacity fade. Different battery technologies display different characteristics in each respect.