Storing electricity From Wind Turbines In Batteries

Once you have a wind turbine sitting on a tower you can start making electricity, but you may soon find that you need something to store electricity in for days when,the wind does not blow. If you want to use the wind turbine to heat water with an electric immersion heater, you could have a well insulated hot water storage tank such as you might use with a solar collector, but if, as seems more likely, you plan to use the wind turbine for lighting, radios and other small loads you will need to store the power it produces in the form of electricity. The only way to do this at present is to use batteries.

Choice of battery

If we leave out the exotic batteries that you will not be able to obtain, such as the high-temperature sodium-sulphur type, you are left to choose between batteries that you fill with sulphuric acid (acid batteries), and those that you fill with potassium hydroxide (alkaline batteries).

Car batteries are always of the lead-acid type, and being mass-produced they are also the cheapest. But they are designed to be portable— which is not necessary for a wind turbine battery — and to allow very large short-term current drains to operate the car starter motor. The main problem with lead-acid batteries when used with wind turbines is that they can be damaged both by over-charging and by total discharging. A set of batteries used to store electricity for purposes such as lighting could easily be fully discharged during periods of low wind speeds. Ideally a set of lead-acid batteries should have twice the storage capacity that you think you want, so that with luck they will never become fully discharged. Twice the capacity means twice the price, and it may be cheaper to have a small diesel generator to charge up the batteries if they begin to get a bit low.

Alkaline batteries — the two available types are nickel-iron and nickel-cadmium — are bulkier than lead-acid batteries, but this does not matter if you are not trying to fit them into a car. Their advantage is that neither total discharge nor overcharging damages them; but they are at least twice the price of lead-acid batteries because they are not mass-produced for the motor industry.

Government surplus suppliers are the best source of batteries, and they often advertise their wares in the Exchange and Mart. Batteries made for military and other government purposes are much more robust than car batteries and will last longer. They are sold unused and sealed, ready for their first filling and charge, and are very much cheaper than car batteries. If you are lucky you may be able to buy ex-government alkaline batteries, and if you can afford them these will last up to twenty years. It is also worth contacting new building sites. If a building is being replaced or modernised they may have a set of batteries to dispose of.

The ideal size

To get some idea of the size of battery you will need you should be aware of your daily electricity demand, and the longest period you are likely to have too little wind to allow the wind turbine to charge the batteries. Let’s say that you will not have any useful output at wind speeds less than 4m/sec. You can contact the nearest meteorological office weather station or RAF airfield for an estimate of the longest period when the winds are less than 4m/sec: now size your battery to cater for this period.

Suppose you want to use a wind turbine just for lighting. If the wind turbine produces 24 volts you could use 24 volt DC fluorescent lamps, which use less current than incandescent filament lamps. Each lamp needs about 0.5 amps, so if we assume you use five lamps and they run for five hours a day the total demand will be 0.5 x5 x 5=12.5ah (amp hours). A typical battery set might consist of four 6 volt 100ah government surplus batteries, giving a storage of 100ah at 24 volts (or 2.4kWh, as volts x amps=Watts), so the batteries would last for 4.1=8 days. If you find that the periods of low wind speeds are not greater than 8 days you will not run out of stored electricity; but if the batteries are lead-acid you should double the capacity to 200ah to lessen the chance of complete discharge.

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