Battery storage of solar energy is a reality, and the economics are just about there. Phil Kreveld reports.

Storing the sun’s energy, which rates about one kilowatt per square metre for much of a clear-sky day, is an idea whose time has come.

As yet domestic, grid-connected photo-voltaic (PV) systems don’t have this storage capability. However, commercial PV systems with battery storage are already in place.

A good example of the latter can be found at the Bega Valley Shire Council library, which is powered by 20kW peak Suntech panels on the roof.

The system can supply three-phase power for the library, storage in a large battery bank and feed-back to the grid at predetermined hours and power levels.

The project was encouraged and part funded by Essential Energy, a NSW Government power distribution company supplying electrical energy for about 95% of regional and rural NSW.

When the utility requests power, the battery bank via the inverter supplies it. In the case of a brownout or power failure, anti-islanding isolates the system from the grid, and the batteries supply the library load via the inverters.

Battery storage opens up an exciting field – micro-grid clusters, which are already employed in Europe. They basically allow disconnection from the grid, and are used to a large extent for DC reticulation rather than AC.

The EMerge Alliance, a US-initiated group of participating manufacturers, provides products for DC lighting systems. DC has an advantage over AC, which in Australia has typical transmission losses of 5.5%.

The California-based Electric Power Research Institute (EPRI) reports that using DC distribution can reduce PV system capital costs by up to 25% by eliminating the inverter and increasing system efficiency.

Equivocation by the Federal Government over renewable energy targets, the rising electricity tariffs, and the half-hearted (at best) attitude to feed-in tariffs have all set the scene for more battery storage solar PV.

Battery storage as an alternative to grid-connected solar systems has a great advantage. It answers the claim by conventional power generators that they need to have ‘spinning reserve’. The more solar installations supplying power into the grid, the more spinning reserve is required, because in cloudy weather the feed-in reduces dramatically and there is an immediate call for additional power.

The spinning reserve argument will disappear the more battery storage comes into vogue, whether connected to the grid or stand alone, and the change will eat further into mainstream energy generation.

The recent decision by the Rockefeller family to withdraw investment in fossil fuel and other conventional energy generation in favour of renewable energy sources will give a substantial boost to energy storage technologies. Storage technology covers lead-acid, nickel-metal hydride, lithium and newer methods (gas-accumulator fuel cell).

However, the economics of storage are generally not favourable yet.

There is an inverse relationship between the depth of discharge (DOD) of a battery and its life, so the energy requirements in a solar PV system during adverse weather is a crucial cost factor. If the PV system is out of action for long periods, the deep discharges of an ‘underweight’ battery would make the system uneconomic.

Putting some flesh on the bones: a valve-regulated lead-acid battery at 80% DOD, has a life of 600-plus cycles. In addition, there is self-discharge of about 4% a month.

The economics of battery storage are hazy. Accountants are notorious for being able to come up with figures that will prove just about anything, and this is very much the case for alternative energy (witness the Warburton Report).

Over the life of the battery, coulometric efficiency is the crucial factor (coulombs are amp-seconds). The solar system (or windmill) puts in a total electric charge and the efficiency factor tells us the fraction of charge recoverable. Lifetime plays an important role, influencing the eventual cost of a kilowatt-hour delivered by the battery.

Critics argue that batteries in general are expensive, and at the moment they are often right. The shorter the life span, the more of the battery cost needs to be recovered .

However, poles and wires, and their recoverable cost, are increasingly loading up the cost of electricity. For a typical householder the generation cost is only 10% to 15% of the tariff.

German research into lead-acid batteries (by Deutsche Energieversorgung) comes up with a storage cost of US21c per kilowatt-hour, to which the cost of generation must be added.

The lithium-ion battery used in electric vehicles is also being employed in energy storage. The German Centre for Solar Energy and Hydrogen Research says lithium-ion batteries are approaching the cost of lead-acid, and may soon be lower.

A brilliant example is Southern California Edison’s Tehachapi 32MW/h storage facility using lithium-ion batteries, with a project cost of about US $50 million. Over two years the project will demonstrate the performance of lithium-ion batteries in system conditions. It will also highlight battery storage system automation and integration into the utility grid.

A more modest example can be found on the Hawaiian island of Kauai. A lithium-ion battery storage system with a 4.6MW/h capacity will provide network stability and mitigate fluctuations that can occur with renewable power sources. The system will also regulate the distribution bus voltage and act as spinning reserve.

The economics of domestic rooftop solar are a little uncertain. Initially very attractive gross feed-in tariffs made the purchase decision for householders easy. Now peak rates ‘when the sun don’t shine’ and feed-in tariffs of 10c per kW/h (or zero) have changed things.

In the past, a six-panel system (1.5kW to 1.8kW) could export some power during ideal conditions and generate about 10kW/h (in theory – when everyone is at work there’s little energy consumption).

A six-panel system accounts for about 50% of daily domestic use in a capital city dwelling. To cater for total energy use, 10 or 12 panels would be required, plus a battery storage system.

At the University of New South Wales, the Australian Energy Research Institute (AERI) is investigating energy storage systems in a $40 million project based on lithium-ion batteries (and perhaps other developments in due course).

Using computer modelling, the AERI and the University of Queensland will examine energy storage and the effects of large-scale adoption on the South Eastern Australian grid.

AERI research fellow Dr Baburaj Karanayil makes a number of points regarding energy storage.

At present, large-scale storage using lithium-ion batteries is considerably more expensive than conventional thermal power. There are also impediments to the adoption of large-scale lithium-ion battery banks, including current sharing in parallel strings and cell temperature problems. However, he thinks these problems will be solved.

Smaller-scale battery banks suitable for domestic rooftop solar use are reliable and affordable, and Dr Karanayil foresees their much wider adoption in Australian cities.

He says a solar PV-battery installation would add perhaps 6% to the construction costs of a dwelling, and the system would have a working life that spanned a mortgage period of, say, 20 years. This would allow a home to be isolated from the grid except for a 10A line to the suburban distribution network.