How can i store the power generated from using an exercise bike?

is it possible, i would like to produce some free electric

Usual way would be to drive an alternator, or a dynamo. An electrical output from an alternator would need to rectified to be stored in a battery, but as the alternator is usually more efficient, any losses incurred because of rectification are usually more than counted for by the "better" alternator.

As to storing the energy... Rechargeable batteries would be the norm. Just like how early woollen mills were powered (by unreliable steam engines) you could use the bike to (directly) pump water/air to a higher level/pressure. When power was needed, you let that fluid flow back to "normal" by driving a turbine (which then generates the electric). Flywheels do store CONSIDERABLE energy, but they are subject to friction (as is everything that moves) and slow up if left alone, they're best suited to very short term energy storage (a minutes or two max)

Not sure how good an "engine" a human makes. think the BEST racing cyclists can manage very SHORT busts of about 1kW. For average person probably would be looking at about 200W sustained power out.

The electric only appears to be free, you still have put effort/fuel in to the engine. Sure I read somewhere that the best cyclists have muscles which are 20% efficient, at converting "food" into "power". Joe average will be a lot less efficient. Given that majority of internal combustion engines are at least 22% eefficient, it means for the SAME output power, an internal combustion engine is more efficient, uses less fuel, and produces LESS Co2, than a human, producing the same power! Humans still produce some pollution, which is left after the fuel is burnt, but nature/our bodies are fairly well geared to deal with it. (CO2 from an engine and from an human is treated EXACTLY the same)

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I've have to compare the cost per watt of power output, of food and petrol, and see which ends up being cheapest.

Consider that even the world's finest bicycle riders (i.e. Tour-de-France, world-class) can only generate about 250 Watts, average, sustained for a few hours. At 4 hours that's 1 kWhr of energy. You and your exercise bike might be able to sustain 200 Watts for 1 hour if you are in good shape. That's 0.2 kWhr of energy. So what are you planning on doing with that 0.2 kWhr?

You can run a 13W CFL bulb for 15 hours, but not much else. With the inefficiencies of the inverter to charge the battery and produce the 120VAC, that 15 hours drops to, maybe, 10 or 11 hours.

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great question...ive thought about that before ..i was thinking bout shoes watches etc to reuse the energy in my body too - but it is the cost involved for the actual amount of electricity stopped me from going any further - i dont know anything about that stuff so cant cost it!!hit a brick wall!

Connect it to a generator and store the output in a battery. Or alternatively use it to charge a flywheel....

nothing is free... you buy food to fuel your body...

but any way, get a generator attach it to the fly wheel and attach the output to some energy cells...

but i bet it'll cost more than its worth.

You will need an alternator, an inverter and a battery. These will not be cheap, so consider whether it is worth it!