This article from the experts at NICEIC looks at some of the component parts that may be employed in an electrical energy storage system (EESS).
Before we start, it should be noted that such types of storage media may be connected in various configurations to the host installation and/or any embedded generation. Manufacturer’s instructions should always be consulted.
EES system components
Any EESS consists of a number of component parts, including:
– Charge/discharge control equipment
– DC/DC conversion equipment
– Thermal management equipment
– EESS control equipment
– Independent earthing arrangement (where required)
– Monitoring and metering equipment (not discussed here).
All of these components may be arranged in a series of modules comprising several of the individual functions. Their functionality is summarised in this article.
EES system types
Within the scope of BS 7671, that is operating at a voltage not exceeding low voltage, an EESS may be categorised as a:
– residential EESS, or
– commercial and industrial EESS.
An EESS may be designed and built for a particular application or it may be assembled from a range of component parts to provide the desired functionality.
A self-contained EESS system is one in which the components have been matched and assembled at the factory and is ready to be installed on delivery to site.
Although exact terminology used in the various standards and guidance documents varies, an EESS may be constructed from a number of subsystems, including:
– the control subsystem containing the communication, management and protection functions and the user interface, and
– the primary subsystem containing the energy storage and power conversion functions and the point(s) of connection to the electrical power system.
Batteries: types & characteristics
The two most commonly employed battery types in an EESS are deep-cycle lead-acid and lithium-ion.
The following characteristics and factors will have an influence on choice of battery type employed:
1. Operational lifespan.
This can be affected by a number of factors including:
– The frequency of discharge, and
– The depth of discharge (DOD) (greater DOD generally equates to shorter battery life)
– Characteristic calendar life (projected life irrespective of usage)
– Overcharge tolerance (lithium-ion batteries are more prone to damage than lead-acid).
2. Required effective/usable energy storage capacity.
3. Charge/discharge efficiency.
This can be affected by factors such as:
– Battery type
– Ambient temperature
– Battery operational temperature
– Battery age
– Frequency of maintenance
4. Required recovery time.
This is the time needed by an EESS, for the specific operating mode and operating conditions, to recover from a duty cycle so that it can be sufficiently recharged to serve the following duty cycle.
Generally, lead-acid batteries perform less well across most of the above categories. However, they may still offer the most cost-effective option in some cases based on the nature of the system and operational expectations.
The arrangement of individual batteries or cells to create batteries/battery banks in order to achieve the desired terminal voltage and output current can vary. A system containing battery storage but no local generation is defined as a battery energy storage system (BESS) (see Fig 4).
Inverters: types & characteristics
An inverter is a static semi-conductor device (power converter) which converts DC to AC. Inverters often include additional functionalities, discussed later in this article.
A number of types of inverter may be employed within an EESS to permit:
– Grid connection
– Standalone operation (without grid connection)
– Combination grid/standalone operation
– Bi-directional operation, which can allow output to the grid and/or the installation and also act as a charger for the storage batteries. Both grid connection and standalone bi-directional inverters are available.
It should be noted that where an EESS is installed at a premises with an existing PV system designed to operate in parallel with the DNO supply only, and is not forming part of a prosumers low-voltage installation (PEI), the associated inverter will shut down when loss of mains or fluctuation in the supply characteristics outside of nominal parameters is detected.
If any form of continued battery charging and/or operation of loads within the premises is required a second inverter or other power conversion equipment (charger/charge controller) will need to be installed or the existing inverter could be replaced. This may be seen as the favourable option where the inverter has been operating for some time, as inverters are often the first component to fail and may be near end of life.
Battery charger/ battery charge controller
In an EESS, an inverter/battery charger is a two-way device which converts:
– DC power from the batteries into AC to supply loads within an installation, and
– AC energy into DC energy to charge batteries.
A charge controller sends power in one direction only to charge the batteries whilst also preventing current from draining back into the source of supply.
Inverter/chargers are not designed to charge batteries directly from the DC current provided by an unstable source such as, for example, a PV system. A charge controller is needed to match the PV output voltage to the battery and to regulate charging.
In general, an EESS will contain both an inverter/battery charger and a charge controller to ensure optimum functionality and efficiency.
In the case of an EESS utilising a PV generator, there may be insufficient capacity to charge the storage batteries effectively using a charge controller alone in the winter or during extended periods of cloudy weather. This can affect energy availability and so an inverter/charger will be required to keep batteries adequately charged from the grid supply.
A DC/DC converter makes it possible to:
– Raise/step-up battery output voltage
– Provide a constant DC output voltage regardless of fluctuations in battery output voltage
– Provide galvanic isolation of the battery from the rest of the EESS to provide, for example, corrosion mitigation.
It can also simplify the matching of multiple battery sets to the EESS.
Battery discharge controller
A discharge controller can provide the following:
– Control of the rate of discharge
– Prevention of further discharge of batteries when their depth of discharge (DOD) limit is reached (that is, the percentage of total capacity that can be utilised)
– Feedback on battery temperature to optimise efficiency and/or minimise damage
– Control of the times at which batteries can discharge to loads
– Prevent discharge until battery charge thresholds have been reached.
The service life of batteries forming part of a bank can be shortened considerably by a charge imbalance.
A cell or battery having even slightly higher internal leakage current in a bank of several series connected cells or batteries will cause undercharge of the whole battery in relation to the other cells or batteries to which it is connected in series, which may also be subjected to overcharging.
Left untreated this can set up a cyclical passage of current between batteries or cells and a resultant temperature increase or thermal runaway. Overcharging can cause damage due to excessive gassing. In the case of L-Ion batteries the aforementioned effects can result in an explosion and/or fire hazard. For lead-acid batteries, undercharging can cause sulphation of the batteries/cells with the lower initial state of charge.
Thermal management system
Although varying in complexity, thermal management systems maintain the component parts of an EESS within their normal operational temperature limits to maintain functionality and efficiency and to prevent thermal damage. At the most basic level, this could be a thermal cut-out. In more complex systems, particularly where continuity of supply is essential, it might trigger cooling equipment to start and/or instigate some degree of non-essential load shedding.
Power conditioning equipment (PCE)
This term is often used where multiple components/functions such as inverters, battery chargers and controllers, DC/DC converters, battery discharge controllers and thermal management equipment are integrated into a single unit.
EESS management system
This may monitor the availability and quality of the connected sources of supply, control charge and discharge operations and may also interface with external resources such as those of distributors and suppliers. It may also instigate other actions such as making/breaking the connection to a public distribution network, load shedding, information exchange and provision of information to building user/operator.
Independent earthing arrangement
An EESS operating in Island mode; that is, where supply to all or part of an installation is maintained from internal sources although the supply from the grid has been disconnected, cannot rely on the earthing arrangement provided by distributor when running independently of the DNO supply (5188.8.131.52.1) as doing so could place persons working on the distribution system in danger.
A typical EESS consists of a number of component parts designed to provide a system which is both economical and remains reliable for its predicted service life. An EESS may be assembled from a number of components or sub-sets, or may be manufactured as a self-contained system.
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