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 discharge rates.

Lead 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.

A 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 mains/generator. 

This leads to small, but continuous electrical losses within the system and reduces the environmental benefit for micro CHP applications.

For complete grid-independence, systems are available from a number of suppliers including Victron Energy.

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 power.

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" efficiency.

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

CHEMICAL                SYSTEMS

 PNEUMATIC

 CAPACITOR

A 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 customer load. 

Whilst 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 systems, 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 of components.

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.