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78 E L EC TR I C AL CONNEC T I ON
W I N T E R 20 1 6
households will install solar and battery systems
over the next decade or so, with a payback
time of less than 10 years.
CSIRO, and even the Australian Energy
Market Operator, have released forecasts that
about 30% of households or businesses will
have their own battery systems in the not too
distant future.
The opportunity
Broadly, there are two ways that a grid-
connected house or business can use
batteries to reduce electricity costs: energy
arbitrage and solar storing.
Arbitrage means buying something when
it is cheap, and selling it when it becomes
more expensive. Batteries allow a customer to
arbitrage energy by charging from the grid when
electricity is cheap, then discharging to run the
loads when electricity is more expensive.
For example, the battery system could
be charged at midnight, taking advantage
of six-cent grid power. Then, later in the day
when electricity is at 50-cent peak rates, the
household wouldn’t need to buy expensive
electricity from the grid.
Solar storing solves one of the conundrums
with solar generating systems. They produce
their maximum output in the middle of the day,
when in many houses nobody is home and the
electricity load is relatively low.
Batteries can save the solar energy
generated in the middle of the day and make it
available later when the household load is high.
An example of this is shown in Figure 1.
Such a scenario is also described as using
the battery to maximise ‘self-consumption’ –
making sure all the energy from a customer’s
solar PV system is used to benefit that
customer, rather than supplying the grid (and
earning very little for it).
But battery systems don’t just benefit the
local household or business. They can offer
widespread benefits to the broader electricity
grid as well, improving power quality and
reliability and even reducing electricity costs for
those without a battery system.
This helps to avoid the installation of the
expensive new poles and wires required to
meet peak demand.
Benefits could come from the aggregate
response of a large number of small battery
systems (a utility might control the batteries in
people’s houses in exchange for a lower power
bill), or by installing a few very large battery
systems at key points in the network.
Both approaches are being trialled by
electricity utilities in Australia.
The technology
The battery systems now being deployed
are a far cry from the large bank of 12V lead-
acid wet cells that made up stationary battery
systems just a few years ago.
Today’s systems are self-contained and
maintenance free. The main components are:
s
The inverter/charger. Previously based on
large, expensive low-voltage (24-48V was
common) transformer-based inverters.
Today’s transformerless inverters operate at
much higher voltages.
s
The battery cells. Today’s cells are
maintenance free. They are designed to
operate across a wide range of states of
charge and many thousands of charge
cycles (often 5000 cycles or more). the most
common cells are lithium based (a variety
of lithium-type batteries exist), or advanced
lead-acid (using new cell technology to
match the performance of lithium). Other
up-and-coming cell technologies include
zinc bromine, vanadium redox and sodium-
ion technologies.
s
Battery management system. This is usually
split into two components – a battery
management system on the cells that
ensures they are not excessively charged or
discharged, and intelligence in the inverter/
charger that communicates with the
cell-level management to ensure optimal
battery performance.
s
A two-way electricity meter (usually an
extra meter operating in addition to the
tariff meter for the site). In operating modes
such as solar storage, the battery system
needs to ‘know’ whether the local site is
importing or exporting energy from the grid. It
determines this through a two-way meter that
communicates with the battery.
The most common combined battery and
solar systems are DC coupled. The batteries
and solar panel are connected on a DC bus,
and the inverter/charger interfaces this to the
electricity grid.
Another approach starting to appear involves
AC coupled battery and solar systems. The
batteries and solar have individual inverters and
are linked at the AC bus of the property.
AC coupled battery systems are
particularly well suited for retrofitting a
battery to an existing solar PV system, as
the battery can be added independently of
the existing installation.
Adding a DC coupled battery to an
existing installation often requires the inverter
to be changed.
The available battery cell technologies (from
lithium to lead-acid and even sodium-ion) have
various advantages and disadvantages. We’ll
provide a detailed study of each technology in
a future article, but for now it’s sufficient to say
that there’s no single ‘winner’.
When considering cost, operating
temperature range, power rating, depth of
discharge, safety and even recyclability, the
technologies have different attributes. They
are suited to different applications, and careful
consideration is essential to get beyond the
marketing hype of some manufacturers.
One important consideration regarding
battery system technologies is how well
integrated the components are. In some
systems the battery module is a separate
box from the inverter/charger and other
“What we once considered a ‘fact’ is
now a myth, and large-scale electricity
storage is possible and economical,"
says Glenn Platt of CSIRO.