Energy Storage

Electrical Storage
Batteries are the most common form of electrical storage. Where heat, rather than electricity, is the desired end product of a wind turbine application, hot water is the usual storage medium. However, the advantageous economics of other heating systems make wind-powered heating a less attractive option.

Batteries can store and deliver only DC power. Unless an inverter is used to convert DC to AC, only DC appliances can be operated from the stored power. The battery voltage must be the same as the voltage needed to run the appliance. Standard battery voltage is 6 or 12 volts. For an appliance requiring 24 volts, two 12-volt or four 6-volt batteries connected in series are required. For a 120-volt application, you will need a series of ten 12-volt batteries.

The least costly batteries for wind applications are deep cycle, heavy-duty, industrial type lead-acid batteries, such as those used in golf carts and forklifts designed for high reliability and long life. They can be fully charged and discharged, while standard lead-acid batteries (e.g., automobile type) cannot. Gel-cell lead acid batteries have improved the safety of the traditional liquid acid battery by containing the hydrogen that can be produced during charging, and by preventing the liquid acid from spilling.

Battery conversion efficiency is approximately 60% to 80%. A battery’s capacity is rated in amp-hours, a measure of its ability to deliver a certain amperage for a certain number of hours. For example, for a rating of 60 amp-hours, 3 amps can be delivered for 20 hours. Batteries should be routinely inspected for fluid level and corrosion. The storage room should be well ventilated. If allowed to accumulate, the hydrogen gas produced by some batteries can explode.

Substantial research on battery technology has taken place since 1990, when the Federal Clear Air Act Amendment prompted new battery research for electric vehicles. Much of this research has focused on developing batteries that can be rapidly charged and are lighter in weight. Many new types of batteries are being developed. Two examples of near-future alternatives to lead-acid batteries are nickel-cadmium batteries and nickel-iron batteries. Both types generally provide good low-temperature performance and long life, but they are still more expensive than lead-acid batteries.

Heat Storage
When heat is the desired end product, hot water is an alternate way to store energy. It is well suited to northern climates where the heating season coincides with the windy season. There are two basic ways to produce heat from a wind turbine. Electricity can be sent to resistance heaters immersed in water, or the wind turbine’s rotor shaft can be mechanically coupled to a paddle or pump that agitates water, thereby heating it.

Resistance Heaters
The first method of heat storage involves electrical resistance heaters which can be DC or AC powered with unregulated voltage and frequency levels. Thus, the buyer has considerable flexibility in choosing a machine without the need for additional complex and expensive control or conditioning devices. The conversion efficiency of a resistance heater is nearly 100%, and heat loss is minimized if the water storage tank is well-insulated. Resistance heaters can also be used directly to heat air, as with baseboard electric home heaters.

Mechanical Heating
The second method of heating water is by mechanically agitating it, using either a pump or a paddle. The heat is produced by the large frictional losses that are produced by agitation. This method of heating does not require an electrical generator. Instead, the power from the rotating rotor shaft is used directly. Theoretical conversion efficiencies are nearly 100%, but practical considerations can reduce this considerably. As yet, only a few experimental models of this type of wind system have been tested.

Sizing of Off-Grid Systems
Sizing remote systems is substantially different than sizing a wind system for utility interconnection because remote systems must be designed to supply the entire electrical demand. Before one can size the components of a remote system, one must determine the load requirements of the site. This means quantifying the power demand on a daily and seasonal basis. The goal is to compare the amount of energy needed at different times of the day and year to when it is available on average from the wind. After taking into account the wind’s intermittence, you can determine the size and type of energy storage or other energy sources needed to meet your total demand.

Battery storage should be sized large enough to handle at least three windless days. Back-up generators are often included in remote electrical systems as a supplemental backup. They help to power large, infrequently used loads and to preserve the life of the batteries by minimizing the number of times they are completely discharged. Also, they run most efficiently at full load. For these reasons, generators are often sized larger than the average expected load of the system so that they can also charge the batteries at the same time, keeping run time and fuel consumption to a minimum. Many wind system dealers are familiar with remote system designs and can assist you in selecting an optimum, cost-effective system.