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# solar/wind powered buggy

#### ilovelamp

Dec 7, 2011
2
Hi,
For a project I need to combine solar and wind power to drive a buggy on a 7m course into oncoming wind.

Before the race we have 5 minutes solar charging time with 2 panels either in series or parallel under 1x 100W lamp. This energy can be stored in any arrangement of capacitors that we desire.

During the race, we must have a wind turbine fixed to the buggy which is capturing the energy from the oncoming wind and combines with the energy stored in the capacitor to drive the motor.

I was wondering what our different options are in terms of how we actually combine the voltages from the capacitor and the wind turbine?? I.e. how we would design a combinational circuit which uses both power sources. Or a microcontroller which uses both.

Any help would be kindly appreciated.

Many thanks.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
It sounds like you have 3 power sources:

1) solar
2) wind
3) stored charge

Further it sounds like the solar only charges the capacitors before the race.

Do you have to carry the panels on the vehicle?

So the task you have is to manage the charge stored in the capacitors until the end of the race, and also manage the power from the turbine.

My strategy would be to use a MPPT regulator to charge up the capacitors as much as possible, then use the MPPT tracking regulator to take the max power from the turbine as possible to further charge the capacitors.

I would use a microcontroller to monitor the charge and to try to exhaust it as close as possible to the end of the race. That means determining your progress (i.e. measuring distance traveled)

You probably want to determine how much power you will need to store and pick suitable capacitors (perhaps supercapacitors). It's probably most efficient to store your power as a highish voltage and use PWM on the motors to control power. The inductance of the motors means that you can limit current by manipulatng the on time of the PWM, so you don't (probably) need any form of switchmode regulation until the power in the capacitors falls below the nominal motor voltage (you could then start a boost SMPS to get the last bit from the capacitors)

You need to ensure that any advantage you might gain from smart electronics os not removed by the increase in weight.

Presumably the conditions will be sufficiently controlled that you can do your own runs and optimise stuff before the event.

#### ilovelamp

Dec 7, 2011
2
Thanks for the reply,

We don't have to carry the solar panels on the vehicle.. Our thought is there will be minimal gain from ambient lighting during the race, and so it will be better saving the weight, and having the panels as a separate charging unit which we can connect to the capacitors.

We have a £100 budget, and MPPT regulators seem to retail for £100+. A possibility could be using a PICAXE 18X as I believe this has pule output abilities. This has the benefit of avoiding gearing a motor down (quite inefficient?). But I am unsure of what kind of % we would want from the motor. I don't have any experience of using pulse width modulation, would you be to advice a good source to start with or explain what SMPS is?

I was under the impression we can't exceed the nominal voltage of the motor, do we not need to run the motor at a voltage under this (say 12V)..?

And yes, the conditions are pre-determined, the race will be carried out in a wind tunnel with oncoming wind at 3m/s. We have to travel 4m, turn 45degrees and travel another 3m to the finish point.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
An MPPT tracker is simply a SMPS that is controlled in such a way that it delivers maximum power to the load. Typically the output voltage is close to fixed, so it tracks max current, but when charging capacitors, the voltage will change continuously, so an actual power calculation would need to be made.

As this is fairly low power, you could probably make one yourself for a fairly low price, essentially an inductor, a mosfet, a diode, a couple of caps, and a microcontroller. The time spent programming this is free

You monitor the voltage on the input capacitor, and on the output capacitor, and across a current sense resistor. The topology would be boost (most likely). You initially set a very low mark/space ratio, and set the increment direction to +ve.

You read the output voltage and current and calculate power, (*) then you increment the mark/space ratio by a small amount in the indicated direction and measure the power again. If the power is lower, you chanve the direction of the increment. Then you go back to (*) and keep looping.

Since your conditions are somewhat fixed (three phases -- charge from light, direct into wind, 45 degrees to wind, you could just read input current and track to a certain value (determined experimentally) your charger would need to know when to change from one to the other, but that's not a big problem.

You also need to decide whether you want your capacitor voltage to be above or below the solar panel voltage. I'd tend to go for above (smaller capacitance, higher voltage) rather than lower (higher capacitance, lower voltage) because it's probably easier to use a buck rather than boost arrangement to power the motors.

Another question you need to ask yourself (or determine experimentally) is whether the power gained from the wind turbine is worth the additional drag it will generate. There may be some utility in just letting it free-wheel if you can get enough power from the solar panels to complete the course quickly. This may be a case where getting maximum power from it is not the best option (but minimum power might not be either).

Yes, if you're using PWN, you can drive the motor from a much higher voltage than normal. Presumably you're only driving the motor forward, so you can have a diode across the motor to complete the circuit when the PWM is turned off (and thus reduce the inductive kick). Because the motor is essentially an inductor, a high voltage does not immediatly cause a high current to flow, the current ramps up. By controlling the mark/space ratio you can vary the peak and average curent flowing.

The cheapest method of gearing is to select an appropriate wheel size. Remember also that large wheels will generate higher wind resistance, and in the face of the wind you're talking about, I would consider that aerodynamics could be an issue.

Do you have access to the wind tunnel to do testing? How much wind tunnel time do you have?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Oh, and MPPT is essentially

power source --> SMPS --> load

The SMPS is controlled so that the max power is transferred to the load. Typically the load has a close to constant voltage, and the power generated varies, so the MPPT device tracks the output current (and tries to maximise it)

In your case, the input power is likely to be constant, and the output voltage will vary (due to capacitor charging), so you can probably track a *fixed* input current (fixed, but different in each of the three phases (solar, head on, 45 deg)).

Basically, you would measure the current, compare it to a reference value, and increase or decrease the mark/space ratio to try to keep it as close to this value as possible. With a boost SMPS you also have to be careful you don't saturate the inductor...

Oh, it would be relatively easy to use a SMPS to produce a (say) 12V rail from the capacitors, then use PWM to control the motor, but if you can eliminate the SMPS, you will improve efficiency significantly (as well as save weight)

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