Page 46 - Energize October 2021

TECHNICAL

Battery storage (ESS) systems The lifetime of VRLA batteries can be affected by the average state
Many rooftop solar systems are now incorporating storage batteries, of charge (SOC). If the SOC average is low, chemical changes can take
which are becoming an essential part of many RPS systems, and failure place which reduce the capacity significantly (Figure 5). Reversing
of the battery can have severe consequences. The main maintenance these changes is difficult and it is important to ensure that the
function which can be performed, other than inspecting the physical battery receives full charge and discharge cycles regularly. This can be
condition, is monitoring the capacity of the battery. determined for the charge control monitor. Constant operation at a low
charge level may be the result of mismatch between the battery and
Two types of battery are used with RPV systems: the PV system or may result from loss of capacity of the PV system. This
• Sealed lead acid or VRLA batteries: These are generally found in may also result if the battery is sized on its final EOL capacity rather
smaller or older systems range. than initial capacity.
• Li-ion batteries: Becoming more popular with the introduction of
home ESS systems. Maintenance and monitoring systems
Most RPS do not have the luxury of maintenance tools such as
Batteries serve several functions: thermographic analysis or soiling monitors and must thus rely on other
• Smoothing the output of the solar array methods of fault or failure detection.
• Standby in the event of grid failure Monitoring and control systems designed for solar PV systems are
• Energy storage for use outside of solar hours and during low solar, usually the cheapest solution to control and evaluate performances
for energy arbitrage, or for EV charging of solar systems, and there is fortunately a wide variety of monitoring
systems of varying degrees of complexity available to the user
The standby application has become very important in the light of today, which will enable faulty or underperforming components or
routine load shedding. faulty conditions to be identified. The systems can monitor both the
Battery failure means either total failure or loss of capacity in production and usage of solar electricity on a daily or even hourly basis
excess of a predetermined amount. Failure can be due to either a fault and provide a useful means of indicating whether action is needed.
in the battery or normal capacity loss to EOL (end of life). All batteries
lose capacity due to ageing and usage and will eventually fail. PV system monitoring
Batteries are generally guaranteed to achieve a given lifetime under Although separate systems are available, many inverters and charge
set conditions of operation. Changes in these conditions can result in a controllers today incorporate systems which provide basic monitoring
variation of lifetime. Solar batteries used with rooftop PV are designed and recording of generation and consumption results, so the cost need
for a nominal lifetime of ten years, and this is usually the guarantee not be considered exorbitant.
given. The actual lifetime of battery can be anything from 5 to 15 years,
depending on how the battery is used. The guaranteed lifetime will There are three types of monitoring system.
generally relate to the number of cycles at a specified depth of discharge. • Stand-alone monitor
The guarantee will specify the capacity at EOL, and this can range • Inverter integrated monitor (includes hybrid)
from 80% for VRLA, and 70% to 60% for some Li-ion battery systems. • Battery charge controller monitor.
This means that at the low end a battery with an initial capacity of 6,4
kWh will drop to about 3,78 kWh within the guaranteed lifetime. This Stand-alone systems
is important as such a drop of capacity may not be acceptable to the Stand-alone systems vary from simple systems which monitor the PV
system, and the lifetime to an acceptable capacity will be shorter than performance only, to complex units which monitor the performance
the guaranteed lifetime. of the electrical system as a whole, including the grid connection
The capacity of the battery is affected mainly by the number and and the ESS. All systems offer some form of remote monitoring and
depth of charge/discharge cycles, (or the total kWh cycled through the recording, ranging from mobile apps to cloud based systems which
battery for some products) and this factor is taken into account when can access a centralised systems database, containing information on
designing the system. Most rooftop PV systems will exhibit a single weather and performance of other systems in the neighbourhood.
charge/discharge cycle per day of a set depth. Daily deep discharge of Stand-alone systems can offer the user more facilities than
the battery, for instance where energy shifting or EV charging regimes integrated systems.
are used, will shorten the battery lifetime.
To achieve the design lifetime, the battery needs to operate within Inverter integrated systems
a state of charge window as illustrated in Figure 5. These systems are integrated into the inverter and provide information
on the performance of the solar array and its components. The level of
monitoring can cover:
• Individual panels: expensive and only necessary with larger
installations
• Individual strings: useful for multi-string installations
• The complete array: the most economical solution for smaller
installations

Charge regulator integrated systems
When separate charge controller and inverter are used, these can
provide information on the performance of the PV array and the
Figure 5: Battery operating window (Ultrabattery) battery, as well as SOC and charge/discharge cycle information.

energize | October 2021 | 44