considering types of electrical storage, it is important to understand
the exact application; whether it is for power conditioning, short term
or longer term storage. Larger scale systems often consider the
application of energy storage for very short periods, either for power
quality purposes or as a short term buffer. Examples include the
Regenysis system developed by Innogy in the UK to be installed within
the distribution system for peak-lopping purposes, or the URENCO flywheel
storage system which has an extremely high charge/discharge rate making
it suitable in applications such as regenerative braking/power support
as used for trains in the New York subway. On a somewhat longer
term basis, pumped hydro can store significant amounts of energy with a
discharge rate in line with hydro generation.
In domestic applications, the most
relevant technology for stand-alone operation is a conventional battery
system (as increasingly used with PV systems). However, in general, it
may be assumed that, for grid-parallel operation, the grid itself is the
"battery" as regards mass electrical storage.
Some manufacturers are considering the
inclusion of very small electrical storage components to provide sufficient
power to start the engine in the case of grid failure (so-called
"black start" capability) and possibly as a load balancing
mechanism for use during grid failure so that a separate UPS
(Uninterruptible Power Supply) circuit can be maintained at all
times. Such a circuit might for example power lighting, heating
ancillaries, TV and refrigeration. This particular feature,
although somewhat unnecessary in UK and most of Western Europe, still
has an appeal to many technophiles and those who want some
"independence" from the central supply system.
Other potential technologies include
compressed air storage and (super) capacitors. The former can be rechargeable
and act as emergency back-up power, whilst the latter tend to be used
for very short term storage with their characteristic high charging and
acid and other rechargeable batteries have been the standard electrical
storage medium for decades. They have a relatively low initial
cost, but maintenance costs are high and performance tends to fall off
throughout their life.
From an environmental perspective, there
are also issues relating to recycling and disposal of components.
Principle battery technologies at a scale
suitable for micro CHP applications are either traditional lead-acid or,
increasingly, Lithium Ion batteries which are developing rapidly as a
consequence of demands from the automotive industry.
However, lead-acid batteries remain
competitive for stationary applications where bulk and weight are less
important than life and capital cost.
UPS comprises an electrical storage component together with the
necessary controls, including a charger and an inverter to convert the
stored DC into AC for use in the home. Depending on the requirements of the
user, the UPS may store only sufficient power to safely power down
computers and other vulnerable equipment, for continuous powering of
emergency appliances for an extended period, or the total loads of a
selected number of appliances (lighting, TV etc.) or even the entire
domestic load for off-grid systems.
In order to provide seamless back-up
power suitable for computer protection, it is necessary to maintain
simultaneous feeds from the battery (through the inverter) and from the
This leads to small, but continuous electrical
losses within the system and reduces the environmental benefit for micro
For complete grid-independence, systems are
available from a number of suppliers including
As a development from simple UPS systems,
there has been a recent emergence of battery storage systems for
residential applications, driven by two key market developments.
In Germany, where modifications to the FIT
which now rewards exported power at a reduced level, there has been a
move towards optimising self-consumption of on-site generation.
The capital cost of the storage device (typically in excess of €10,000)
is recovered from the difference in value between imported and exported
In Japan, where for different reasons export
from fuel cells and other micro CHP technologies is not viable,
the same driver supports the economic case here as well. However,
the key driver in this market arises from the crisis of confidence in
the national electricity supply system following the disaster at
Fukushima and the subsequent reduction in available power capacity.
It is difficult to quantify this perceived value, but it appears that
Japanese consumers are prepared to pay upwards of €60,000 for an
integrated PV, Fuel Cell, UPS system which provides reliable continuous
power to their homes.
In addition to the points noted above
concerning the desired storage characteristics (power, discharge rate
etc.) it is also worth considering the reversibility of the storage
process. All storage systems suffer some loss of energy during
charging, storage and discharging, referred to a "round-trip"
One option promoted by some as a very low
cost option is to dump excess electricity into, for example, the
existing hot water cylinder in the home using a simple resistance
element (immersion heater). In this way it is possible to store
say 10kWh for less than €100, a fraction of the cost of electrical
storage. However, clearly this cycle is not reversible so devalues
the exergy significantly, to an equivalent value of the grid primary
energy efficiency, typically around 40%, although this could of course
be enhanced if a heat pump were to be used, but with the inevitable
capital cost implications.
In some respects this is the exact opposite
of the process described in the section on thermal storage where a
primary thermal store is used as a (reversible) proxy for electrical
generation from micro CHP.
flywheel energy storage system draws electrical energy from a primary
source, and stores it in a high-density rotating flywheel. It is
effectively a kinetic battery, spinning
at very high speeds (>20,000 rpm) to store energy that is instantly
available when needed.
Upon power loss, the motor driving the flywheel acts as a generator,
supplying power to the
URENCO system was designed for
large power applications,
Beacon Energy have developed products which,
although still too costly for typical domestic applications, are viable
for niche applications such as remote telecoms stations.
Flywheel-based energy storage systems,
unlike lead-acid batteries, are relatively “green” technology
solutions that do not use hazardous materials for production, nor create
them during operation. Unlike batteries, flywheels operate reliably for
many years with little or no maintenance.
Despite higher initial costs than battery
flywheels offer an attractive, cost-effective energy
storage capability where rapid response to high power (relative to
stored capacity) is important..
Chemical storage is similar in concept to that of a fuel cell, in that
chemicals are fed directly into the regenerator to produce electricity.
However, unlike a fuel cell, the chemicals can also be processed back
into their former state when electricity is supplied.
One particularly interesting aspect of this
technology is that the power rating is independent of the energy stored
so that large chemical storage can provide extended power at a rate
constrained only by the conversion device.
As with battery storage, however, the chemicals involved are hazardous
and consideration needs to be given to recycling and eventual disposal
The production of hydrogen from low carbon
electricity and the subsequent reconversion into electricity by fuel
cells is a form of longer term energy storage; hydrogen also has the
valuable characteristic of being suitable for utilisation in other
applications such as a replacement fuel for internal combustion engines
in the automotive sector.
Compressed air storage has been successfully demonstrated at utility
scale using underground caverns.
At a scale closer to domestic there are now
products available which store energy in the form of compressed air in
cylinders which may either be charged from the system when power is
available or, as shown here in the Energetix PnuPower system, using
replaceable compressed air cylinders. whilst this reduces the
capital cost and complexity of the system, it also constrains the energy
storage to that provided by the limited number of cylinders and requires
manual intervention to replace the cylinders once depleted.
Capacitors are capable of rapid charge and discharge, but are not suited
to long term storage.
Even the so-called "super" and "ultra"
capacitors are not suited to storage of significant amounts of long term
energy storage, but are increasingly being used in combination with
batteries to provide both rapid charging and long term capability,
particularly relevant for automotive applications.