I am designing a small energy harvester/measuring unit. A propeller is placed in a stream of water and will act as a energy harvester. It should be able to generate power enough from the flow to send data twice a minute through an Xbee
The propeller will be rotating at about 50RPM - this is obviously not changeable.
Hardware:
I will be using an arduino Fio - it seems to have a charging circuit and Xbee is easily mounted. I will add a 1000mAh battery to have some power in reserve, should the water stop flowing.
Xbee pro to send data
The general approach i thought about taking was to connect a rectifier circuit to the alternator to get a DC voltage. Then to a DC/DC converter that regulates the voltage to > 3.7V (compatible with Arduino Fio) and then directly into the Arduino Fio since it will take care of charging the battery.
Question 1) How does that plan sound???
Question 2) HOW DO I CHOOSE THE ALTERNATOR????
I am aware that i can use a BLDC motor, but how do i determine how large it has to be? It would be great if i could use a fan from a PC - knowing that the flow is almost constantly such that i will have 50RPM on my propeller i dont need to charge in bursts but rather a lower constant charge. But
Question 3) I obviously need some kind of gearing since the low RPM wont give me any voltage from the alternator. But how many RPM will i be aiming for - would i want to aim for half the no load speed of my motor/alternator as the speed/power characteristic plot of a motor indicates?
The propeller will be rotating at about 50RPM - this is obviously not changeable.
It might be obvious to you.
Boat designers rarely start with the propeller. Why are you?
should the water stop flowing.
Well, that will certainly change the propeller speed. But, that's obviously not allowed.
HOW DO I CHOOSE THE ALTERNATOR????
Quietly, please. You need to have some idea what current and voltage you want. Then, you compare that to what the alternator is capable of producing at some speed. Then, gearing is a matter of picking gears that will turn the output shaft at the right speed, given the available input speed.
Of course, that means that the propeller needs to be capable of generating the needed torque.
@PaulS: Okay okay okay, here is the entire story - i guess i have to be happy since people seem to be paying attention to the small details
It is not a creek but flow in a pipe - i know there is more or less constant flow but form time to time the flow is stopped in order to maintain the pipes and the area they deliver water to. It is not for long but long enough for me to need a battery. Initially i have to design the harvester/logger for this system, but over time it will be implemented on other pipes with other velocities and dimensions.
I have already been looking at the propeller design, but the velocity is so slow (in the order of 0.5m/s) and the pitch of the propeller will be about 20 deg. I dont think is is very efficient to get lower then that - therefore the 50RPM. I know that i might need to implement a diffuser but i would rather not due to several factors.
Regarding the DC motor jremington, then i know about the relationship you are describing but i would rather use a BLDC due to maintenance (brushes) and noise (spikes from the commutator) in the system.
Regarding the energy needed i have calculated that it will be in the region of 2Wh/24h (on average 0,083W because the system is in sleep mode when not transmitting wirelessly)
Correct me if i am wrong but i was planning to use the dc/dc converter to boost my voltage to the desired in order to charge. Stepping up the voltage will reduce the amps but that is how it has to be. I know that the available power in a stream of water is
P=0,5density_of_waterA*v^3=1,3W
Cp is Betz limit = 59%, v= velocity of medium
It can be shown that the maximum theoretical power one is able to extract is dictated by Betz limit - therefore it would be 0,8W
BUT (not yelling)
The thing is that the calculation is for flow in an open canal - here we have water in a duct. The harvested energy will be the pressure drop over the propeller multiplied with the volume flow [m^3/sek]. But since i don't know what the pressure drop will be and i cant measure it i am a bit lost.
It is not for long but long enough for me to need a battery.
How long? If it is a one-shot experiment, why not just use an automobile battery with an efficient DC-DC converter? That could conceivably power the devices for months.
What is the diameter of those pipes? And what is the "engineer level" the device should look like? If DIY or homemade is acceptable, I would look around me if I have some DC motors and try it with them. If it should be more professional, I can't advise anything
Maybe I'm missing something obvious but a few complications occur to me
Small DC motors (or BLDC, for that matter) won't generate much energy at 50 rpm. And gearing up will entail huge inefficiency in a small system.
What pressure difference is acceptable across the turbine? If you want to generate energy from the flow in the pipe you will inevitably restrict the flow - perhaps well below half its free flow rate - even if you had a magic turbine that doesn't reduce the cross section.
Because the turbine must reduce the cross section you will probably need to fit it into an enlarged section of pipe so that the larger cross-section less the turbin cross-section is the same as the original pipe cross-section.
On the other hand if you just want a device to measure the flow rate ....
It is supposed to be a permanent installation so the engineering level is high. Battery is a good idea but out of the question. It is inside so no sunlight for cells. Regarding pressure drop, no limit but i dont want to be ridiculous. It is in a production facility so there is a LARGE pump connected (but unfortunately i cant pull power from a socket). But, when i do the math (pressure drop multiplied with volume flow) the available power becomes high quite quick. It is in SI-units so the pressure is in Pascal (100.000 to the Bar). Just to put it in perspektive - If 1/10 of a bar (10.000Pa or roughly 1m loss of head) could be introduced, you could theoretically harvest 138W (which is about 70 times more then i need) if i am not mistaking.
Regarding the gearbox i was planning on using a belt drive with a ratio. I may need to make it in several steps, but there is a very high efficiency associated with belt drives - 95%ish.
Regarding the diameter of the pipe, it will vary but on the pilot project a 200mm.
Kawgomoo:
you will need to see 4000-6000rpm at the alternator shaft for good output.
I guess that depends on the size of the alternator. The small air-core motors from eg Maxon, have a no load speed of tens of thousands RPM, but down to 1200RPM can also be found in their catalogue.
The reason i would introduce a venturi is due to the equation i showed earlier: P=0,5rhoA*v^3 because of the term v^3 so i would perhaps crank that up by reducing the cross section hereby increasing the velocity and reducing the pressure.
