Should I choose to add batteries instead of getting grid connected?

Batteries are used in off-grid or charging applications. Off-grid systems are employed when the cost of connecting to the grid is prohibitive or the customer chooses to remain off-grid. As off-grid systems don't have the back-up of the national grid to call on during times of low or no wind they need some form of storage if they are to have a constant source of electricity.

The capacity of the storage can be just enough to smooth out the short term variances in wind availability or it can be enough to provide power to the premises if there is no electricity produced by the turbine for periods of days. The amount of energy stored depends on the size of the generator installed, how long the customer wants to satisfy their on-site demand and on how much they are willing to spend on battery cells.
Batteries are an arrangement of cells. Cells can be purchased separately (e.g. 2 Volt) and can be assembled to create batteries of almost any required voltage (e.g. 24 Volt).

The type of batteries used with wind turbines are known as 'deep cycle' batteries (or deep discharge). These are not like car batteries which are designed to deliver a large current or be called on to deliver a large amount of energy over a short period of time, for example during ignition.

There are variations of deep cycle batteries which are less suitable for long slow charges and discharges such as 'leisure' and 'semi-traction' batteries. Full 'traction batteries' are the most suitable type of deep cycle battery and are the most commonly used in off-grid systems. They are the type used in fork-lifts and submarines. Electric fork-lifts are typically charged slowly overnight and depleted during the day which is close to the likely operation of an off-grid power system.

There are environmental issues to consider when looking at batteries. Much of the battery material can be recycled but elements such as lead and cadmium are toxic and their use should be minimised in the first instance even if they can be recovered. Batteries can be dangerous (explosive acidic fumes, liquid acid, shorting of terminals) and every precaution should be taken in safe storage and ventilation. Professional expertise should be employed.

Control of the charging and discharging of batteries is crucial if the batteries are to last a reasonable length of time. An expensive battery bank can be damaged beyond use if not employed correctly. Batteries need to be accompanied by a wind turbine specific charge controller to prevent damage as manual control with meters and switches is not a realistic option for most people. Most batteries are designed to deliver only a portion of the stored energy and not be discharged fully under normal operation. Full discharge will shorten the life of the battery considerably.

For grid connected wind turbines where the user has back-up electricity available in times of low or no wind, albeit at the retail price per unit, adding batteries is sometimes considered. It is considered when electricity suppliers will not pay for excess electricity exported on to the grid. If no payment is available then customers may wish to store the electricity for use later. The economics of such a decision need to be carefully considered however in light of the decision by ESB Networks and ESB Customer Supply to pay some categories of customer for exported energy. Payment to the remaining categories of customers is under consideration. When the export tariff is equal to or greater than the import tariff the grid effectively becomes a storage device- you sell to grid when you don't need it and buy it back when you do. When the export tariff is greater than the import tariff then there is a clear financial benefit to being grid connected over using batteries.

Battery storage is expensive and a well maintained bank may last only half as long as a good quality, well maintained turbine. Depending on the size of the turbine installed and the amount of storage required batteries can add €2,000 to €5,000 to a domestic scale system. Second hand deep cycle traction batteries have become increasingly difficult to source.